The sexual expression or breeding system for tropical populations of Arisaema macrospathum was compared with that of its temperate counterpart, A. dracontium. Although A. macrospathum populations are morphologically very similar to A. dracontium, they differ markedly in sexual expression. Temperate populations of A. dracontium are andromonoecious, while those of A. macrospathum occurring in cloud forest were found to bear only unisexual flowers, staminate or pistillate with monoecious individuals completely lacking. Male plants produced a mean of 42.7 flowers/spadix while females produced a mean of 60.9. For males, number of flowers/ spadix showed a significant, positive relationship with plant size as measured by basal stern diameter, but no relationship was detected for females. Non-flowering and male plants did not differ in size, but were significantly smaller than female plants as is typical for the congeneric A. triphyllum and probably switch gender under similar circumstances also. Considering the entire geographic range, A. dracontium exhibits a gradual decline in plant size, number of flowers/spadix for males, and an increase in the expression of female plants along a north-south gradient. This gradual decline in size would appear to extend into populations of A. macrospathum and suggests a close evolutionary relationship. Differences in sexual expression between the two species may be related to geographical isolation and the presence or absence of competing congeneric species.