Long-term inapparent infections were established in continuously grown uncloned Aëdes aegypti mosquito cells inoculated with Semliki Forest virus and incubated at 28°. This response of mosquito cells to infection, which contrasts with the outcome of infection by the same virus of mammalian cells at 37°, was due to a small proportion of virus-producing cells in the culture, and a low yield of virus per infected cell.