The reproductive ecology of river lamprey Lampetra fluviatilis was investigated during the spawning period 2003 in the River Derwent, north‐east England. Over this period 1199 ± 104 individuals day−1(mean ± s.d.) were counted on one spawning site (area c. 450 m2), but mark‐recapture estimates suggested that >5000 river lamprey used this site over the same period and egg deposition was estimated as 168 000 eggs m−2. The operational sex ratio of river lamprey in spawning clusters changed between spawning phases, from domination by females during the nest‐building phase (male : female ratio, 1 : 3·46), to a preponderance of males during the spawning phase (male : female, 1 : 0·37), followed by a return to a majority of females after spawning (male : female, 1 : 3·74). Recapture data showed that >97% of recaptured, tagged males were recorded at two or more nests, whereas almost 50% of recaptured, tagged females were recorded at the same nest, suggesting a promiscuous mating system, with a tendency towards polygyny within the population. Within the lower 80 km of the River Derwent and its tributaries, evidence of river lamprey spawning was found at only six sites, and most spawning (>80% of the observed spawning population) was at one site.