The reproduction and early growth of the erpobdellid leechNephelopsis obscurain laboratory culture are presented, enabling a comparison to size structure of natural populations. IndividualNephelopsis obscurain culture were found to produce cocoons over a prolonged spawning period with emergent young of markedly different weight. These characteristics along with differential growth of the young result in a highly skewed weight distribution of a generation of leeches. The extreme weight range in a cohort ofN. obscurapresents difficulties in aging erpobdellids.