Technical Abstract: The alfalfa leafcutting bee, Megachile rotundata (Fabricius), is commonly used to pollinate alfalfa, Medicago sativa L., for seed production in the U.S. and Canada. It is difficult to reliably sustain commercial populations of M. rotundata in the U.S. due to problems with disease, parasites and predators, and unexplained immature morality. One possible explanation for early immature mortality is that superfluous numbers of these bees are released in fields where resources become limited. For two nesting seasons (2003 & 2004), we performed an experiment in field enclosures to determine if pollination efficiency and reproductive success of M. rotundata is density-dependent. Our results showed that relatively high numbers of foraging M. rotundata could not be sustained and never reached maximum establishment potential in the field enclosures, and effects on seed yield could not be measured because of poor field conditions and thus seed production. However, we found significant reductions as bee density increased in the percent of pollinated flowers per female bee and the number of nests and cells produced per female. We also found an increase in the percent of early brood mortality in the form of pollen balls with decreasing bee density. The number of progeny that lived to adulthood, regardless of progeny sex, was negatively affected by relative bee density. This study shows that bee reproductive success is density-dependent, but open field studies are needed to determine what commercial bee densities can result in sustainable bee reproduction levels while at the same time effectively pollinate alfalfa for profitable seed production.