We describe aspects of the life history of the small cardinalfish Ostorhinchus rubrimacula from a single, large collection taken at Koro, Fiji. We determined size at maturity and batch fecundity, examined otolith microstructure to construct a vonBertalanffy growth curve, described a length-weight relationship, and performed a dietary analysis. Ostorhinchus rubrimacula is a gonochore that matures at 35 mm standard length (SL). Batch fecundity is related to body size by the equation: eggs spawned = 0.0013(SL)^3.8685. Assuming each otolith ring corresponds to 1 day in age, the oldest individual in our collection lived 274 days. Growth is described by the equation: SL = 40.84[l − e^{−0.014(age in days − 22.45)}]. Total body mass (mg) = 4.806 · 10⁻⁶(SL)^3.5163. Ostorhinchus rubrimacula feeds primarily on harpacticoid copepods, but isopods (mostly gnathidean) and polychaetes were also important dietary components.