Brachiola (Nosema) algerae is a microsporidian species generally believed to be an intracellular parasite of insects, especially mosquitoes. However, both mosquito and human isolates have been shown to infect mammalian cells. The present study was undertaken to determine if spores of two insect and two human isolates of B. algerae cultured at 30 °C and 37 °C differed in their ability to germinate and infect cultured green monkey kidney cells at these two temperatures. Spores from all four isolates exhibited an optimum pH of 9.5 for germination. Mercury (Hg² ) inhibited germination of all isolates equally. Germination of spores from all four isolates was significantly greater when the parasite was cultured at 30 °C than when cultured at 37 °C. However, spores from the insect isolates cultivated at 30 °C or 37 °C infected significantly fewer mammalian cells at 37 °C than did spores from the human isolates under the same conditions. Thus, there is no correlation between the effects of temperature on the germination and the infectivity of an isolate. In addition, while exposure of B. algerae to 37 °C has been reported to cause spore dysmorphism, we failed to observe any consistent ultrastructural changes that explained the greater infectivity of the human isolates at 37 °C.