Effects of rice field water quality and host age on the infectivity ofLagenidium giganteumCouch toCulex tarsalisCoquillett were investigated in the laboratory for 3 yr. Selected water chemistry parameters were quantified to determine their influence on the corresponding infection rates. Large variation in dose-response was obtained among different water samples. When individual instars were exposed to a concentration of 10 zoospores/ml in different water samples, infection for the first through fourth instars respectively averaged 49, 42, 56, and 47%, with 78% of the variation in larval mortality significantly correlating with changes in common water chemistry parameters. Singly, none of the water chemistry variables accounted for>35% of the variation in infection rates. The causal relationships between changes in water chemistry and infectivity were unclear. When all instars were bioassayed in the same container to remove variation caused by different water samples, the first three instars were equally susceptible to infection, and the fourth instar was less susceptible. With exposure to 100 zoospores per ml, the period of lethal infection increased from 18 to 48 h as host age increased from the first to the fourth stadium.