Infections of mammals by species in the phylum Oomycota taxonomically and molecularly similar to known Lagenidium giganteum strains have increased. During 2013-2014, we conducted a phylogenetic study of 21 mammalian Lagenidium isolates; we found that 11 cannot be differentiated from L. giganteum strains that the US Environmental Protection Agency approved for biological control of mosquitoes; these strains were later unregistered and are no longer available. L. giganteum strains pathogenic to mammals formed a strongly supported clade with the biological control isolates, and both types experimentally infected mosquito larvae. However, the strains from mammals grew well at 25[degrees]C and 37[degrees]C, whereas the biological control strains developed normally at 25[degrees]C but poorly at higher temperatures. The emergence of heattolerant strains of L. giganteum pathogenic to lower animals and humans is of environmental and public health concern.