The objectives of this study were to determine the effects of acidification on 0+juvenile fish diversity and abundance in River Kyrönjoki in 1995 and 1996. Species diversity and abundance data were obtained from seine hauls in aquatic vegetation biotopes of nymphaeids predominantlyNuphar lutea.Acidity levels decreased steadily from May to August in 1995. In 1996, acidity levels increased during May, July and early August. Acidification levels were related to low abundances of 0+juveniles, except for 0+juvenile(Esox Incites L.)in the estuary, and lower species richness in the 0+fish community in the river in 1996. Single species dominance, expressed as rank abundance curves, was typical in the river's 0+juvenile fish community during the acidification in 1996.