The use of olfactory cues for nest recognition by Osmia lignaria and Megachile rotundata was studied in greenhouses in Logan, UT during 2003-04. The objectives were to determine if females of both species mark their nest and if so, to locate where the marking occurs within the nest. Each greenhouse section contained an observation room. Glass tubes were used as nesting cavities, so that in-nest behavior could be observed. Each glass tube had been cut into 3 sections: an outer section (2 cm) opening to the greenhouse, a middle section (4 cm), and an inner section (8 cm) plugged at the end. We observed that nesting females of both species drag their abdomen along the tube before exiting, sometimes depositing tiny fluid droplets from the tip of the abdomen. During manipulations we recorded the behavior exhibited by tested females upon arrival to the nesting site and inside the nesting tubes. Three treatments were conducted in which we removed and replaced by similar clean glass tube sections: 1) the outer section; 2) the middle section; 3) both sections. As a control treatment we disassembled and reassembled the whole tube. The confusion displayed by females of both species returning to nests after non-control treatments clearly indicated the presence inside the whole nest of some olfactory cue used for individual nest recognition. Our findings provide a better understanding of Megachilid nest recognition that could lead to improved commercial-scale bee management practices in both species.