ABSTRACT Mothocyabohlkeorum, new species, is described from the gill chambers of the whitestar cardinalfish Apogon lachneri Bohlke from Puerto Rico; and the dusky cardinalfish Phaeoptyx pigmentaria (Poey) and the freckled cardinalfish P. conklini (Silvester) from the Bahamas. It may be distinguished from Mothocya nana (Schioedte and Meinert) by the shape of the frons and telson; and by having a cephalon which is slightly instead of deeply immersed in pereonite 1. Larger female M. bohlkeorum occur on the larger hosts. Female isopods may undergo vegetative growth stages between reproductive brood stages. Male and female isopods occur in opposing gill chambers of a host. This duplex male-female association may explain how relatively large cymothoid isopods colonize small host species.