For the tanner in the Karroo and the karroid areas the genus Tribulus is of great interest as it includes plants which are at times excellent fodder plants and at other times suddenly prove fatal to sheep causing the dreaded “dikkop “. It is up to now not all clear whether the causing of “dikkop " is confined only to certain species or, what is more likely, whether all the species may under particular edaphic and meteorological conditions produce dikkop. It is unfortunate that the systematics of the genus is not properly worked out, thus it is often impossible to say which particular species is actually found in the veld and whether other plants which have decidedly another habit of growth are different species or only a physiological variety. According to the literature (Theiler 1918, Quin and Rimingt.on 1930, 1934, 1935) the view is more wide spread that it is a question of physiological state, and not of different systematic species. The author having observe the growth of Tribulus on different Yeld (alluvial soil, farm yards, limestone, dolerite brown soil and sandveld) was at first rather inclined to think of different systematic species, with their respective hybrids, especially Tribulus terrestris and T. parrispinus. Since Schweikerdt (1937) most recently puts these two species together again, and only accepts terrestris, the view had to be abandoned and in the present paper the different varieties will be called physiological strains.Cultivation experiments which are in progress on the Veld Reserve at Fauresmith ought to bring about a final conclusion.