The New Zealand freshwater crayfish P. zealandicus emerges from water and exposes it's respiratory surface to desiccation and collapse, it's haemolymph to acid-base disturbance and it's tissues to hypoxia. This study was to investigate the respiratory system during aerial respiration. P. zealandicus has 20 gills, a rudimentary gill and an epipodite (20 + r + ep). Two types of gill filament were identified; a respiratory filament with a cuticle 0.7µm thick, an afferent and efferent vessel and haemolymph lacunae adjacent to the cuticle, and an ion regulating filament with a cuticle 1.2µm thick in which the cells adjacent to the cuticle contain all the organelles and membranes which are associated with ion regulating tissues. It was found that P. zealandicus emerged from the water voluntarily. There was no significant difference in the frequency of emersion by solo crayfish at 18°C, crayfish pairs at 18°C, solo crayfish experiencing hypoxia at l8°C, and solo crayfish at 24°C. Solo crayfish and crayfish pairs recorded a significantly higher level of emersion activity at night than during the daytime. At 15°C the settled rate of oxygen consumption in air, 1.03± 0.03 µmol.g-1 .h-1 (± 1 sem), was not significantly different from the settled rate of oxygen consumption in water, 1.10± 0.03 µmol. g-1. h-1. Oxygen consumption in water was compromised by declining external oxygen tension below PCR1T (41 - 44 Torr). At PCR1T the oxygen consumption was 1.06 ± 0.05 /lffi01.g-1 .h-1 At only one time in 48 hours aerial respiration was haemo1ymph arterial oxygen tension below PCRIT .This was after 12 hours aerial respiration, and was associated with an increase in lactate concentration to 5 mmol.1-1 , which was, however, removed during the subsequent 12 hours aerial respiration. The crayfish were not considered to be experiencing internal hypoxia for the remainder of the 48 hours in air. The total oxygen consumed during 8 hours recovering from aerial respiration was similar to the total oxygen consumed during the 8 hours settling at the beginning of the experiment, indicating no measurable oxygen debt accumulated during 48 hours aerial respiration. After emersion into air the crayfish experienced a respiratory acidosis, and an elevated arterial carbon-dioxide tension. Total haemolymph carbonates, measured 12 hours after emerging from water indicated compensation by metabolic alkalosis. Upon returning to the water the crayfish experienced a respiratory alkalosis. Rapidly declining arterial carbon-dioxide tension, and total haemolymph carbonates indicated compensation by metabolic acidosis. The measured in vitro non-bicarbonate buffer capacity was -3.8 mmol.-1 . (pH unit)-1. During the 48 hours in air the pH was regulated between 7.55 and 7.68 with the pH significantly more alkaline at 8 pm than at 8 am, by 0.1 pH units. At 15°C, the solubility of oxygen in haemolymph, ±PLASMAO2, was 1.8 µmol.1-1.Torr-1, and at an oxygen tension of 150 Torr the total haemolymph oxygen content was 1 mmol.1-1. There was a Bohr effect, ΔlogP50/ΔpH, of - 0.48 to - ,0.96. Settled in water at 15°C, with a haemolymph carbon-dioxide tension of 2.6 Torr, the crayfish haemocyanin was 50 % saturated at an oxygen tension of 12 Torr. Crayfish kept at 15°C had an oxygen content when the haemocyanin was 50 % saturated, and a total oxygen content, which were 50 % higher than crayfish from water at 9.5°C. Estimates of limitation to diffusion/perfusion, Ldiff, in water of 0.76, and in air of 0.98, indicate that respiration in P. zealandicus is diffusion limited. The respiratory frequency, fR, and heart frequency, fH, of settled crayfish in air was not significantly different from settled crayfish in water. The fH during aerial respiration demonstrated less fluctuation than fH in control crayfish in water, and the fR of crayfish in air was more variable than the fR in control crayfish in water. The control crayfish settled in water exhibited a significant diel fluctuation in fR and fH, with high fR and fH recorded in the evening and low fR and fH in the morning. Several activities and changes measured in this study have a diel rhythm. A dusk pH recorded at 8 pm. was 0.1 pH units more alkaline than a dawn pH recorded at 8 am. Settled in water, P. zealandicus exhibits a diel fluctuation in fR and fH, with high fR and fH recorded in the evening and low fR and fH in the morning. At 18°C, solitary crayfish and crayfish pairs show higher levels of emersion activity at night than during the daytime. The results indicate that crayfish P. zealandicus has a respiratory system which can cope with aerial respiration, and some of the activities and changes reported in this study have a diel rhythm.