Weight change of alfalfa weevil larvae infected with anEryniafungus was compared with that of noninfected larvae at different saturation deficits. As expected, the developing fungus was generally sensitive to changes in atmospheric water concentration. Weight loss of infected larvae decreased as saturation deficit levels decreased. Typically, percent weight loss of infected larvae was>percent weight loss of noninfected dead larvae during the ca. 7-h period between rhizoid emergence and initial conidial discharge. Above 1.5-mm-Hg saturation deficit, infected larvae in the weight tract experiments formed rhizoids and pseudocystidia but not conidiophores. The data suggested a water loss threshold (based on weight change) for conidiophore emergence in the 1.5- to 1.8-mm-Hg saturation deficit range. Overall, infected larvae lost more water than did noninfected dead larvae. This was primarily due to loss of cuticle integrity and increased surface area available for evaporation. Based on these factors and the shape of the weight track for infected larvae, it was inferred that diffusion processes are highly influential to maintenance and development of conidiophore production.