1. The nest-site selection behaviour of the bee Halictus rubicundus (Christ) was examined both within and across sites in the U.K. Females utilized a range of edaphic and microclimatic conditions when choosing a site to excavate a nest. Factors with broad tolerances included slope and hardness; those with much narrower limits included aspect, soil humidity and soil particle composition.

2. There was a preference for softer soils that were easier to dig within a site with a low overall density, but in much denser aggregations problems of maintaining the structural integrity of a nest led to the utilization of harder soils.

3. The thermal advantages of having a warm nest meant that the most suitable areas were those with a southern aspect and a slope that maximized the absorption of solar radiation.

4. Limited areas of substrate with the most desirable characteristics resulted in gregarious nesting (‘limited substrate hypothesis’).

5. Natal nest-site fidelity complemented the ‘limited substrate’ hypothesis in producing an aggregation of nests.