Smittium culisetae, isolated from the hindgut of a mosquito larva, grew in a liquid tryptone-glucose medium over a range of temperatures from 5[degree] to 32[degree] C. The maximum rates of growth were in the range of 20-30[degree] C, but maximum dry weight was achieved at 10[degree] C. When pH values were not subsequently adjusted, the fungus grew best in media with an initial pH of 8.3 or higher, but with appreciable reduction in pH occurring during growth. The ultrastructure of S. culisetae is generally typical of fungi. The hyphal septa, however, have a single pore that is surrounded by a swollen septal area. Electronmicrographs of spores showed that they possess at their base a similar type of pore.