Nosema algerae Vávra and Undeen 1970, a microsporidian known to cause infection in mosquitoes, develops in mammalian cell cultures at 24-35 degrees C and in the tails and footpads of athymic mice. More recently it has been reported to grow at 38 degrees C in human cell culture. The present study is a two-part temperature/development examination. The first part examines the development of N. algerae in rabbit kidney cell culture at 29 degrees C, which permits the formation of functional spores within 72 h, and compares the effect of elevated temperatures (36.0, 36.5, 37 degrees C) on parasite development. At these elevated temperatures, N. algerae infects but undergoes only one or two proliferative divisions, with no evidence of sporogony by 72 h post-inoculation. During this time, however, the host cells continue to divide resulting in fewer infected cells over time and giving the appearance of a diminished parasitemia. Additionally, at 37 degrees C some organisms degenerate/hibernate by 72 h while others remain viable/active. It is not until 96 h that the parasites appear in large clusters of proliferative stages in the few host cells that are infected. By 120 h post-inoculation, proliferative cells, sporoblasts, and early spores are observed. These results suggest that elevated temperatures impede proliferation rates and the onset of sporogony. The second part of this study evaluates developmental changes in N. algerae when incubation temperatures and times are varied during parasite growth, resulting in abnormal parasite morphology. These abnormalities were not present when parasites were grown at constant temperature (29-37 degrees C). This report demonstrates that N. algerae can successfully develop at high temperatures (37 degrees C), justifying its taxonomic relocation to the genus Brachiola.