During an 11 month study 1,479 Australian grayling, Prototroctes maraena, were collected, and the life cycle of the species was elucidated. Spawning takes place in the freshwater mid-reaches of the Tambo River during late April-early May (Fall 1979). The eggs are demersal and non-adhesive. The newly hatched fry are presumably swept downstream to brackish water in an estuary or to the ocean where they remain for about six months. During November juveniles ascend to the mid-reaches of the river where they spend the rest of their lives. Most (88%) grayling die before depositing a third annulus on their scales. The largest grayling examined was 231 mm standard length. Males may spawn during their first year, females require two years in fresh water. The oldest fish collected had five annuli and well-developed gonads. Mature females produce an average of 47,000 eggs and have a Gonadosomatic Index of about 27% near spawning time. Mature males develop breeding tubercles on the scales and fin rays when the GSI reaches 5.7%. As much as 12% of a ripe male's body weight may be testis. The breeding season of this schooling species is short and synchronized: gonadal development begins in mid-March, reaches a peak in late April and declines by early May. The cause of an apparent decline-abundance cycle in grayling populations may reflect its complex life cycle and high fecundity. Several difficult environmental years in succession (typical of Australia) could decimate the grayling population. The survivors could explosively exploit succeeding favorable years.