1 investigated seasonal, lunar and diel patterns in the spawnlng behaviour of a serranid, the common coral trout Plectropomus leopardus, on the Great Barrier Reef, Australia, uslng underwater visual census surveys. The study was conducted a t Scott and Elford Reefs, 2 mid-shelf reefs off Calrns, for 4 and 3 yi-, respectively. At each reef, coral trout aggrcgdted and spawned at the same 'primary' aggregation slte in each year. Primary sltes are defined as those with the largest aggregations of coral trout. Spawning activity, though not lim~ted to, was concentrated a t primary sites. The aggregatlon site at Scott Rccf measured l? 00 m' in area, and that a t Elford Rc.ef 3200 m' Maximum numbers of coral trout recorded in these sites were 128 fish a t Scott Reef and 59 a t Elford Reef. Coral trout aggregated and spawned at the primary sites around 3 consecutive new moons each year within the period August-Decembcr. 1991 to 1993 In 1990 only 2 aggregatlons were detected at Scott Keet, both around the new moon The largest aggregations were recorded at the primary s~t e at Scott Reef: numbers of fish at the aggregation site rose from an average prc-spawning density of 3.9 fish 1000m-' to aggregation densities ranglng from 37.1 to 75.3 fish lOOOm -'. The median size class of aggregating fish was 41 to 45 cm FL (forklength) at both reefs in all years. with a maximum range of 16 to 80 cm FL a t Scott Reef, and 16 to 65 cm FL dt Elford Reef The onset of the spawnlng season col-I-esponded with a rlse in water temperature (>24.00dC) after the austral winter. Spawning aggregations occurred for an average of 5 d , however aggregatlons were not found at the sltes throughout the day. The aggregatlons appeared to disperse in the morning and re-establish after 13:00 h. (:oral trout spawned in pairs predominantly on flooding tides and when current flow at the spawning sites was minimal. Ninoty-four spawning rushes were observed which only occurred during a 33 (i4 SE) min period spanning sunset. The spatial and temporal predictability of P. leopardus spawning aggregatlons makes them vulnerable to overfishlng, but also amenable to specialised management through s e~~s o n a l and/or spatlal closures