We used isozyme markers to obtain an estimate of multiple paternity (mating of females with multiple males) in the unisexual, haploid leafy liverwort Porella platyphylloidea. From a sample of 119 plants, 15 (12.6%) were non-sex-expressing, 16 (13.4%) were females without sporophytes, 38 (31.9%) were males, and 50 (42.0%) were females bearing sporophytes. Thus, 87.4% of all plants expressed sex, and 73.5% of females bore sporophytes. The overall observed sex ratio was female-biased at 1.74:1. We extracted a total of 121 sporophytes from the 50 fertile females. Using the strict criterion that only female plants bearing two or more sporophytes with different multilocus genotypes represent cases of multiple paternity, our overall estimate is 56.8% (46 out of 81). Spearman rank-order correlation coefficients revealed that plants growing lower on trees were significantly larger than those higher on the tree and that female plants were significantly larger than male plants. Multivariate analysis showed, however, that the primary determinant underlying these correlations was the larger mean size of females.