The phreatoicidean isopod genus Crenoicus is found in upland swamps and springs throughout New South Wales and Victoria. We provide a new diagnosis of Crenoicus and review its 4 species, including one new species. As a benchmark for phreatoicidean external morphology, this paper provides a detailed description and illustrations of the new species found on the Boyd Plateau west of Sydney. Variation in some morphological features is high, with populations from separated swamps nearby showing localised differentiation. Measurements of large samples from two sites indicate that this species may reproduce continuously throughout the year, but with a substantial decrease during the winter.