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1. The phenotypic variability of several estimates of fitness among seventeen nesting female 0.lignariu Cresson was examined in a glasshouse provided with abundant resources. 2. Females exhibited wide variation in: (a) number of nests and cells made and in their rate of construction, (b) the incidence of mortality of offspring, and (c) the percentage of male offspring produced (52-100%).3. In confirmation of earlier field studies: (a) more offspring of both sexes were produced during the first half of the nesting season, (b) most female offspring were produced early in the nesting season, (c) most parental investment during the last half of the flight season was made in male offspring, (d) there was a decrease in offspring size with season, with females decreasing more than males, and (e) the ratio of femalelmale body weight was = 1.8. These results appear to be due to ageing rather than to a reduction in resource availability. 4.No relation between female size and fecundity was evident. 5.There was no evidence that small females tended to produce a greater proportion of the smaller sex (males) than did large females. 6. Smaller offspring of both sexes had a much greater probability of dying overwinter than did large offspring. However, where the sexes overlapped in size (large males, small females), almost all females died and almost all males survived. We hypothesize that as size of offspring produced declines with season, a greater proportion of males are produced because they have a much greater probability of surviving at small body sizes.

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