An alternative bioassay using neonate larvae of Aedes aegypti is described for determining the toxicity of suspended particles, particularly insecticidal bacteria, to mosquitoes. This new assay is comparable in precision to the 4th instar bioassay recommended by the World Health Organization, but is quicker and requires much less space. Values obtained using neonate larvae as the test insect showed less variation within and between replicates in regard to LC50 and slope of the regression line, and in general required fewer replicates to fulfill the requirements for a statistically valid bioassay than the W.H.O. assay. The precision of the neonate larval bioassay in combination with its requirements for less time and space may make it especially useful for screening large numbers of samples.