Technical Abstract: Compounds thought to be effective against mosquitoes as either insecticides or repellents have recently been shown to contain properties of both, or possess other behavior modifying actions. Prompted by these reports, we conducted posterior analyses of our data, collected as part of a long-term mosquito toxicant/repellent screening program, to examine interrelated statistical issues inherent in our bioassay system. Using a modified K&D module, over 25,000 adult Aedes aegypti females exposed to either alphacypermethrin or DEET were compared to mosquitoes exposed to untreated (solvent) controls for toxicity and biting (alphacypermethrin), or biting (DEET). Our analyses indicated that (1) based on statistical power, our bioassay system is better suited to determine a compound’s toxicity than its repellent qualities; (2) day-to-day variability is large and needs to be accommodated in analyses; there are other, potentially even larger sources of variability (e.g. mosquito heterogeneity), which, taken together, invalidate statistical tests that are based on the assumption of binomially distributed data (e.g. chi-square tests); and (3) unlike biting mosquitoes exposed to DEET, the proportion of biting mosquitoes exposed to alphacypermethrin were unrelated to the proportion of biting controls, even after adjusting for daily variation in toxicity. Thus, there is a clear behavioral indicator in this bioassay system that the repellency of DEET (a presumed repellent) differs in a fundamental way from that of alphacypermethrin (a presumed toxicant and may allow the differentiation between classes of compounds based on biting behavior alone).