Stenotarsus subtilis Arrow (not S. rotundus Arrow) is the aggregating endomychid beetle from Barro Colorado Island, Panama, subject of many studies on a diapausing aggregation. Allozyme analysis results (Nei genetic distance = 0.005) supported conclusions from morphological study, revealing the true identity of two diapause groups now known within the Barro Colorado Island Nature Monument. They have an the estimated aggregation density of 2.2 per km², with up to 200,000 individuals in each aggregation. Females predominate in these groups, but sex ratios varied from 1:1 to 1:4. One group has persisted at least 20 years. Bark was depleted of Mg and K on both Oenocarpus C. F. P. Martius (Palmae) and Tetragastris J. Gaertner (Burseraceae) diapause sites, but did not differ visibly from surrounding bark. Fungi showed no consistent relationship with beetles or sites. Bark and wood substrates of an aggregation did not attract beetles when displaced to other forest areas. We suggest beetles may live more than one year and return to diapause sites after completing the still undocumented mating and reproductive facets of their life cycle. Stenotarsus rotundus is synonymized under S. ovalis Arrow, new synonymy, and a key is given to known species of Stenotarsus occurring in Panama.