Female-biased sex ratios are often associated with small, isolated populations. These conditions are exhibited in populations of Platyarthrus hoflmannseggi (Brandt, 1833), a small, blind woodlouse which lives almost solely in ant nests. This study was undertaken to determine how sex ratio varies with population size in P. hoffmunnseggi and how both of these factors affect population growth rate. To accomplish this, a total of 2155 of these isopods were collected from 20 nests of the ant LusiusJaour (Fabricius, 1781). The majority of the isopod populations were female-biased. The behaviour and survivorship of the isopods in their own and foreign antcolonies were compared and suggest that the free movement of P. hoffmannseggi individuals among ant colonies may be extremely limited. These studies also show how isopods are nutritionally linked to their hosts. Measurement of the woodlice revealed two distinct size/age classes in each population and allowed the change in sex ratio from one generation to the next to be estimated. Female-biased populations produce in future generations relatively more females and fewer males than populations of a similar size with a sex ratio of unity. There was no correlation between the degree of female bias and the rate of population growth. These findings are reviewed in the light of recent theoretical sex ratio models and the possible control of the isopods sex allocation by unusual chromosomal mechanisms and intracellular parasites.