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The biology of the very early stages in the upstream migration of the River lamprey has been studied using samples taken from the cooling water intake screens of the Oldbury Power Station in the Severn Estuary. Examination of the numbers of lampreys caught at different times indicate that an increase in freshwater discharge is the predominant environmental factor responsible for initiating the movement from the sea into the estuary, although temperature may also be a contributory factor. The migrants could be separated on the basis of size into typical and praecox forms whose mean lengths during peak abundance were approximately 300 and 240 mm respectively, the corresponding weights being about 53 and 22 g respectively. The typical forms were occasionally found in the estuary as early as July and as late as April, with peak abundance generally being reached in November, whereas the praecox forms were present mainly between January and March. The ratio of typical to praecox forms over the four years of sampling was estimated as 3.3 : 1. In both size categories, the gonadosomic and hepatosomic ratio was greater in females than males. Evidence was also found in the typical forms for a correlation between high numbers and an increased proportion of males. Measurement of a number of different characters, including lengths, weights and condition factors, as well as gonadosomic, hepatosomic and gut ratios, suggest that, although the typical forms enter the estuary over a long period of time, the onset of the changes leading to sexual maturity are more synchronous. A small number of the later typical migrants, however, exhibited different characteristics to those of the majority of the animals comprising this size category. Measurements made on typical animals from Oldbury in November indicate that they can regulate their plasma ions in salinities as high as 70% of full strength sea water.