Red grouper (Epinephelus morio) are long-lived, commercially important, soniferous fish belonging to the family Epinephelidae. Found throughout the western North Atlantic and Gulf of Mexico, they are protogynous hermaphrodites, and peak spawning occurs from March through May. Unlike many grouper species, red grouper do not form large spawning aggregations; rather, they form small polygynous groups, and remain in relatively close proximity to rocky depressions excavated in the sandy bottom by males. This excavation activity creates structure and habitat for a wide variety of species, and as a result, red grouper are a keystone species on the West Florida Shelf. While extensive life-history information exists, largely from fishery catches, little is known about sound production or behavior of red grouper in their natural environment. Passive acoustic recordings combined with simultaneous digital video recordings were used to investigate sonic activity and behavior of red grouper on the Steamboat Lumps and Madison-Swanson marine reserves on the West Florida Shelf. Red grouper were found to produce a unique series of low-frequency (180 Hz peak) pulses, consisting of 1-4 brief (0.15 s) broadband pulses and a 0.5-2 s down-swept "buzz" (i.e., short call); occasionally these were followed by a rapid series of 10-50 broadband pulses (i.e., pulse train). Sound production was observed throughout the day and night, but most sounds occurred between sunrise and sunset, with a noticeable increase during late afternoon. Behaviors associated with sound production included territorial displays and courtship interactions, indicating that sound production is likely related to spawning activity. Thus, monitoring red grouper using passive acoustics could be an effective tool in fisheries management and conservation efforts.