The spawning migration of brook lampreys (Lampetra planeri, Bloch) was investigated in a South Swedish lake and stream system for two consecutive years. The nightly variations were analysed by a multiple regression technique. In one of the seasons, characterized by frequent rain storms, water level was the most important factor explaining 60% of the observed variation in migration. In the following year, when the migration period was unusually dry, temperature and on‐shore winds at the mouth of the stream were the most influential environmental variables explaining 50% of the observed changes in behaviour. When data were combined for the two years only temperature showed a significant influence. The critical temperature for the onset of migration was estimated to be 7.5°C. Size distributions and sex ratios of the migrating lampreys, which are semelparous, indicated that the age distribution was complex and that females were older than males. Migration was essentially nocturnal, but late in the season day migration was also observed. A marking experiment resulted in few recaptures probably because of high predation and/or orientation difficulti