Astrophytum coahuilense and Astrophytum capricorne var. senile have partly sympatric distributions in southern Coahuila and adjacent Zacatecas, Mexico. The apparent absence of natural hybrids in areas of sympatry has been noted by several authors, leading to speculation concerning the mechanisms of reproductive isolation. Experimental crosses between the two species revealed prezygotic isolating mechanisms in some cases and postzygotic mechanisms in others. Several different results were obtained from the cross-pollinations: 1) fruit was not produced and the ovules in the ovary of the floral remnant showed no development; 2) fruit contained incompletely developed, mostly non-germinant seeds; 3) apparently normal seed was produced, but many of the hybrid seedlings manifested morphological anomalies and mortality was high (>97%) due to chlorophyll deficiency. A series of experimental crosses between A. coahuilense and several allopatric taxa of the A. capricorne group revealed only postzygotic isolation. The discovery of prezygotic isolation in some trials between the sympatric taxa is consistent with theoretical expectations. Postzygotic isolating mechanisms are costly in terms of gametic wastage and therefore natural selection should favor the evolution of one or more prezygotic barriers between the interacting species. The varied results of the experimental crosses between A. coahuilense and A. capricorne var. senile (postzygotic isolation in some cases, prezygotic isolation in others) are possibly explained by differing histories of interaction between populations of the two species.