Growth variability in extant Peneroplis has drawn attention since early times. There is general agreement that the aberrations in recent populations of Peneroplis are natural and intraspecific. For these reasons, some authors have questioned the systematic relationships among living Peneroplis species. In this study, we explore variation in an exceptionally abundant population of Peneroplis from Pete's Pond, a permanent wetland, within the Lake Macleod Evaporate Basin, Western Australia. The area is unusual because foraminifera are isolated from the nearby ocean, except for a 15-km subterranean karst system that permits a constant delivery of seawater. Pete's Pond water salinities are either less than or at normal seawater levels. The morphological variations identified have been grouped, with morphogroups A, B, and C referred to as variants of the Peneroplis growth pattern, including morphotypes with high flaring and cylindrical uniserial chambers. Morphogroups D and E are referred to as ‘abnormal’ aberrant forms, which show bifurcated serial chambers or a disordered growing style. Variants are the most critical in terms of taxonomic discrimination. Here we recommend the following morphological constraints for species determination of Peneroplis variants: i) degree of involution of early chambers; ii) presence/absence of umbilical depressions, iii) incision of sutures and iv) ornament, including the presence and disposition of small pits or ribs. Aberration in Peneroplis is considered to be linked to strict environmental parameters that have led to optimum shell growth as well as to having a key role in the phylogenesis of Soritoidea through deep time.