The structure of the yellow form of arsenomethane has been studied by single crystal x-ray techniques at reduced temperature. The crystals are monoclinic and belong to the space group C2h5 - P21/n, with a = 8.88 A, b = 12.55 A, c = 11.61 A, and beta = 101°46'. The unit cell contains four molecules. Harker synthesis, Patterson section and Fourier projection methods were used to locate the arsenic atoms. Geometrical considerations were combined with the difference Fourier method to find the carbon atoms. The parameters were refined by application of the least-squares procedure to three dimensional diffraction data. The molecule consists of a puckered, five-membered ring of arsenic atoms to each of which is attached a methyl group. The As-As bond length varies significantly from 2.42 A to 2.45 A, and the C-As bond length varies from 1.94 A to 1.98 A. The As-As-As bond angle varies from 971/2° to 1051/2° and the C-As-As bond angle from 94° to 1021/2°.