Largemouth bass (Micropterus salmoides) are the primary sport fish in Puerto Rico reservoirs and the majority of management efforts directed at this species primarily employ electrofishing in sampling activities. However, little attention has been given to evaluating electrofishing sampling efficiency in these systems. To evaluate differences in diel and seasonal electrofishing efficacy, largemouth bass were sampled in two diel periods (day: 0900-1500 h; night: 2100-0300 h) every three months over a three-year period in Lucchetti Reservoir, Puerto Rico, using a boom-mounted electrofisher. No overall statistical difference was observed between day and night electrofishing catch rates for stock-size (≥200 mm TL) largemouth bass in Lucchetti Reservoir (F = 1.51, df = 3, P = 0.2222), yet night catch rates were greater in 9 out of 12 samples. Monthly mean catch rates were lowest in May and highest in February. Length-frequency distributions differed between day and night samples in 3 out of 4 pooled sampling months (February, August, November; all P ≤ 0.0078). If management objectives target high catch rates of largemouth bass, night electrofishing may be most efficient; however, length bias may be of concern and both day and night sampling may be necessary to accurately illustrate true population parameters.