1. Patterns in abundance and distribution of larval fish in a heavily regulated and a mildly regulated Australian lowland river were compared over four breeding seasons to gain some insight into how river regulation affects fish populations.2. Larvae from a total of 13 species from nine families were recorded from the two rivers. The mildly regulated Broken River supported twice as many species as the heavily regulated Campaspe River. The two rivers shared three introduced species but only two native species. The dominant species in the Campaspe was not found in the Broken River.3. The two most abundant species in the Campaspe were classified as `opportunists'. They are small, short-lived species, which spawn for up to 9 months, encompassing extremes in temperature and flow. The extended spawning period may place a subset of larvae in optimal conditions for recruitment and is hypothesised as being the key to the success of these species.4. Most species spawned each year, despite large interannual variation in flow and temperature conditions. Poor recruitment over several decades, rather than a failure to spawn, is considered the most likely explanation for differences in the larval fish faunas between the two rivers.5. The highly regulated section of the Campaspe River downstream of the regulating impoundment is thought to provide suboptimal habitat conditions for larvae relative to the less regulated downstream sections.6. The timing of occurrence of larvae of the dominant species varied by breeding season and may be the result of flexibility in the timing of spawning.