In comparative tests with two species of microsporidia of the Mexican bean beetle, Epilachna varivestis, Nosema epilachnae was decidedly more virulent than N. varivestis, with the lowest dosage rate of N. epilachnae (1 × 10³ spores per larva) resulting in an adult emergence rate of only 50% in contrast to a 96.3% survival rate of larvae exposed to the highest dosage rate of N. varivestis (1 × 10⁶ spores per larva)—a 1000-fold difference in spore dosage rates. The adults infected with N. varivestis also survived an average of two to three times longer than those infected with N. epilachnae. Although both species produced significant reductions in adult longevity and fecundity, the test individuals were exposed to N. epilachnae as either late-stage larvae or as newly emerged adults while those exposed to spores of N. varivestis were inoculated as neonate larvae. Reductions in fecundity were manifested as lower average numbers of egg masses per adult and egg masses per adult per day; however, neither species reduced the average number of eggs per mass on a consistent basis. Reductions in adult longevity and fecundity by N. epilachnae were related to adult age at exosure. Both species were also transmitted transovarially, but the average rate of transmission of N. varivestis was low and did not increase in time. In contrast, the incidence of transovarian transmission of N. epilachnae increased with time to levels approaching 100% and was accompanied by a dramatic decline in egg hatching rates. The highly virulent nature of N. epilachnae for both larvae and adults of the MBB indicates that further effort to evaluate its potential as a microbial control agent is warranted.