Spawning of Barbus mattozi in the Mtsheleli Dam, Zimbabwe, is highly seasonal and timed to take place with the first floods. In December 1974, the Mtsheleli River flooded after heavy rain and the entire adult population of B. mattozi migrated upstream to spawn during a 24h period. Spawning took place in the late morning and early afternoon with 73% of females having spent gonads by the evening. No further spawning occurred, even though the river flooded periodically over the next few months. The sex ratio of fish captured during the December spawning run was 62 males:38 females, although almost all large fish (>15cm FL) were females. Both sexes reached sexual maturity at about 13cm FL and females carried about 7 000–120 000 eggs in their ovaries (logn = −3.13 + 3.49logFL, r = 0.94, P < 0.01), depending on length. The December spawning apparently failed, as no juveniles were collected in 1975, possibly because larvae were swept into the impoundment, where they could not survive. It followed an earlier failure during the 1972–1973 drought, when the river failed to flow, and highlights the risks faced by potamodromous fish in semiarid environments.