Observations of uniquely marked females of the solitary, twig-nesting bee, Osmia bruneri, were conducted under greenhouse conditions to test several predictions of sex-ratio and parental-investment theory. In support of Fisher's (1958) theory, we found that the observed sex-ratio of progeny in this dimorphic species did not differ from that expected on the basis of average male and female weights. Investment patterns also exhibited a seasonal component: female parents produced more female than male offspring early in the nesting season but reversed this pattern later. Interfemale variability was large for all nesting parameters examined. Neither female-parent size nor the rate at which females completed cells was significantly related to several estimates of parent fitness. Parent-offspring heritability for size was also low. We found no evidence to support the hypothesis that progeny sex-ratios are influenced by maternal condition. Variance in progeny sex-ratios was large, but the population sex-ratio probably departs frequently from the equilibrium value. The results marginally support Kolman's (1960) prediction of large variance in progeny sex-ratios in large panmictic populations. We conclude that variability among females in investment patterns and variability in size among progenies are probably maintained by such factors as resource heterogeneity and the shape of the adult survivorship curve.