Consistent palaeomagnetic data have been obtained from 19 out of 31 sampling sites in the thick Proterozoic dolerite sills which intrude the Roraima sandstones of the Guiana Shield in Venezuela and British Guiana (now Guyana). Fourteen of these sites, most of which are in noritic dolerite, have remanence orientations which fall into two significantly different groups, approximately opposite in declination, but both with positive inclination: group I, N = 7, D = 334, I = +27, K = 29, α95 = 11, N pole co-ordinate = 63 N 129 W; group II, N = 7, D = 145, I = +39, K = 25, α95 = 12, N pole co-ordinates = 45 S, 13 W. The remaining stable sites, mostly from norite which occurs near the base of some thick sills, have scattered orientations. The unstable sites are characteristically in rocks representing later, iron-enriched stages in the dolerite differentiation sequence. The published potassium-argon ages on these dolerites, and minerals separated from them, give evidence of differential argon loss, but all with a minimum age of about 1500 m.y. There is no clear evidence in the radiometric data of the two separate periods of dolerite intrusion implied by the palaeomagnetism. An additional four sites, group III, are from dykes which are known or inferred to belong to a distinctly younger dolerite suite, of uncertain age. The mean orientation of these four sites is: D = 17, I = +21, K = 36, α95 = 16, N pole co-ordinates = 73 N, 11 E. Ilmenite is the exclusive or predominant oxide accessory seen optically in the magnetically stable Roraima dolerites, whereas magnetite is conspicuous in the unstable sites. Magnetite, however, in very small amount, is the only ferrimagnetic phase indicated by bulk magnetic property measurements (Js/T, J/H) of stable samples. It is presumed to be the carrier of the remanence, and its stability as compared with that (much more abundant) in the unstable sites is tentatively attributed to finer grain size. In the stable, younger, group III dolerites, however, magnetite is the predominant accessory oxide.