As part of a project on the etiology of chalkbrood, a mycosis of larvae of the alfalfa leafcutting bee, Megachile I'Otul/data (Fabricius), the conditions of pH and oxidation-reduction potential within the alimentary canal were studied. A series of redox-indicator dyes were incorporated into a pollen-based larval diet. After larvae had fed for 2 days on the treated diet, the colours of dyes were observed before and after dissection. The pH of gut contents was also measured at the same time with indicating paper. Except for slightly alkaline conditions in the posterior midgut, pH values were near neutrality and similar to those found in larvae of the honeybee, Apis mellifera. Redox potentials ranged from + 132 to -266 mY, but were consistently negative in the central portion of the midgut. Low redox potentials in insect guts have been associated with the digestion of wool and cellulose. Although the latte.r is a component of pollen cell walls, it is not known if M. rowl/data larvae are able to digest it. As pollen cell walls apparently pass undigested through the honeybee gut, the significance of the reducing conditions found in the midgut of M. /'Owl/data larvae is not known.