Four mosquito spiroplasmas, Spiroplasma culicicola, Spiroplasma sabaudiense, Spiroplasma taiwanense, and an isolate designated CUAS-1 were nonpathogenic for suckling mice and suckling rats inoculated either intracerebrally or intraperitoneally. S. culicicola persisted as long as 14 days in the brain of intracerebrally inoculated mice but did not replicate. The three other spiroplasmas were not reisolated from inoculated rats or mice. S. culicicola replicated in vitro at 37 degrees C in M1D media. A low but clear-cut immune response was elicited in suckling rats inoculated with S. sabaudiense. S. culicicola and S. taiwanense were pathogenic (life span reduced) for Aedes aegypti females inoculated intrathoracically. The progeny of Ae. aegypti females inoculated with S. sabaudiense had a significantly reduced emergence rate in one of two trials and a significant distortion of the sex ratio in two of two trials (increased percentage of males). Female Ae. aegypti inoculated with S. culicicola had a significantly reduced fecundity rate.