Following 3 years of a 356 mm (14 inch) minimum length limit on black bass, a protective length range (slot) regulation was placed on Arbuckle Reservoir, Oklahoma. This slot length limit protected both largemouth and spotted bass in the 300-381 mm (12-15 inch) range. Bass density, as indicated by spring electrofishing catch-per-effort (C/f, increased over the duration of the study. Most noteworthy was the increase in density of largemouth bass>381 mm long. Proportional Stocking Density (PSD) also improved as an indication of changes in population structure. Initially, angler compliance was low (30% illegal harvest) but improved over the course of the study. Few spotted bass achieved a size greater than the slot and their proportion in the population relative to largemouth bass increased.