The upstream migration of the nymphs of the mayfly P. chelifer Bgtss., in a temporary stream, tributary to the river Vindelalven in northern Sweden, was investigated in May 1977. The tributary drains an alluvial meadow which has usually dried out by the middle of July. The upstream movement of nymphs lasted for about 9 days, during which period hundreds of thousands of P. chelifer moved from the main river up into the small stream. The nymphs on the alluvial meadow grew significantly faster than did those in the river, probably because of a higher water temperature and a more plentiful supply of food in the temporary stream. Before the stream dried up completely the nymphs had emerged. Newly hatched nymphs were found in the main river in Feb. P. chelifer seemingly has a colonization cycle which includes an upstream movement of nymphs and a subsequent downstream return flight of adults.