ABSTRACT: This study measured biological variables necessary for determining the dynamics of spawning aggregations and measured the influences of these variables on effects of fishing on these aggregations. Four major spawning aggregation sites of Plectropomusleopardus (Serranidae) were identified at Lizard Island, Northern Great Barrier Reef, Australia, using ultrasonic telemetry. Spawning aggregation activities displayed a lunar pattern, with peak activities during new moon periods in the australspring-early summer. Of 35 fish tracked during spawning periods in 1993-95, only 31% participated in spawning aggregations, despite all specimens being sexually mature. All specimens that aggregated displayed fidelity to their chosen aggregation site. Thedistance between established home ranges and spawning aggregation sites ranged from 220 to 5210 m, with total spawning movements back and forth for individual fish ranging from 600 m to over 17 km. One-way inter-reefal movements were recorded for 3 fishmoving 3, 7.5, and 11 km between release and recapture locations. Male fish spent an average of 8 times longer at aggregations than females. Females (n = 4) undertook single-day or overnight trips only, while males (n = 8) often did multi-day trips. Maleswere also more likely to make several trips to spawning aggregation sites than females. Since males spent more time at aggregation sites, and made more trips to those sites, they were potentially more vulnerable to fishing pressure on aggregations. Thereliance on several aggregation sites per reef makes P. leopardus potentially less vulnerable to overexploitation of spawning aggregations. However, the strong site fidelity observed makes individual aggregations vulnerable to depletion. The lowparticipation rate of reproductively mature individuals at major aggregation events (31%) suggested that not all spawning activity took place at the known aggregation sites.