The causative agent of chalkbrood in the mason beeOsmia lignaria prop inquaCresson is a fungus recently namedAscosphaera torchioiYoussef&McManus. The present study was undertaken to devise a methodology to measure the infectivity potential of this fungus accurately, based on laboratory experiments designed to assure consumption of all spores by host larvae reared under nonstressful environments. Doses of 10, 100, 1,000, 10,000, and 100,000 spores per cell were tested, and the influence of temperature was measured by exposing replicate treatments to 20, 25, or 30°C. The levels ofA. torchioiinfection obtained in this study (0.36–8.5%) showed a linear correlation with increasing dose, and these low-level infections overlapped chalkbrood rates found in large field populations studied over a 15-yrperiod (0.4–2%). The infection rates reported in this study (0.36–8.5%) are, however, much lower than those induced in a recent series of related laboratory studies (57%). Causes for the large discrepancy of results obtained in this study versus results reported in earlier studies are attributed to previous experimental designs in which host larvae were reared in stressful environments, producing larvae of reduced vigor which resulted in enormous increases of chalkbrood mycosis.