Several temperature-sensitive (ts) mutants of Western equine encephalitis virus (WEEV) have been isolated previously from persistently infected cultures of mosquito cells and divided into three groups: early passage RNA- mutants, early passage RNA+ mutants and late passage RNA- mutants (Maeda et al., 1979(. The growth patterns of these groups, as well as of several ts mutants isolated after chemical mutagenesis and of wild-type (wt) WEEV, have been compared in BHK cells and in two strains of mosquito cells. The late passage ts mutants grew much better in mosquito cells than either the wt WEEV or the chemically induced mutants. When mosquito cells were co-infected with a late passage mutant (A125) and Wt WEEV, infectious virions of both parental types as well as phenotypically mixed particles were produced. Infection of mosquito cells with WEEV resulted in a slight suppression of host DNA and protein synthesis during the acute stage of the infection (the first 1 or 2 days). Virus growth in a line of cloned mosquito in which WEEV produced a cytopathic infection (c.p.e.) was analysed with the result that the viruses could be divided into two groups: one in which wt WEEV, chemically induced ts mutants and early passage RNA+ mutants all induced maximal c.p.e., and another in which late passage RNA- mutants and one early passage RNA- mutant induced very little c.p.e., but released much more infections virus into the culture fluid. Electron microscopy showed that in these cloned mosquito cells infected with a virus of the first group, large amounts of virus accumulated on or in the plasma membrane.