Nest construction in hollowed-out plant stems or abandoned beetle burrows poses special problems for those solitary bee and wasp species involved. Typically they select a nest bore diameter only slightly greater than the body diameter (Krombein 1967, Michener 1970). and thus cannot turn around inside. Moreover, the cells of the nest are constructed in a linear series separated by partitions. so emergence of the next generation must proceed in an orderly progression. Correct orientation at pupation is vital for survival, because a misdirected cocoon not only causes the individual to become fatally trapped, but will often doom its siblings earlier in the cell series. We here report evidence that cocoon orientation by the solitary megachilid bee,Osmia lignariaSay, is primarily determined by reference to the placement of the pollen food store within the cell.