MERCURIALIS ANNUA: is a ruderal of pan-European distribution. In southern Spain, southern Portugal and northern Morocco, males coexist with monoecious (cosex) individuals at frequencies which vary between zero and about 30 per cent. Here, I report the results of two glasshouse experiments which aimed to characterize the mode of sex determination in one such androdioecious population. In a breeding experiment, cosexes isolated from males produced only cosexual progeny, whereas those allowed to mate with males produced both cosexual and male offspring. The results are consistent with a single-locus model of sex inheritance, with maleness determined by a dominant allele. In a density experiment, the frequency of males correlated positively with density, confirming field observations. Differential mortality or germination between the sexes was excluded by experimental design, so that the result supports the hypothesis of density- dependent gender choice. A negative correlation between density and the proportion of 'late cosexes' (males which turned cosexual late in their development), and the lack of any differences in the proportion of pure (unchanged) cosexes between density, suggest that only males were capable of sex change. This interpretation is consistent with the results of the breeding experiment, in which late cosexes occurred only in the offspring of cosexes allowed to mate with males. I argue that these findings help to explain the maintenance of androdioecy in M. annua.