Occurrences of nymphs ofSeratella tibialisin streams of eleven western states were related to water quality and physical habitat parameters of stream stations. Nymphs were collected from only 182 of 903 stations sampled in nine states; the species was absent from all 50 stations in two other states.Seratella tibialisnymphs occurred mostly at stations with good water quality and stable stream banks. Absence of nymphs has limited predictive value since the distribution ofSeratella tibialiswas naturally spotty and the species was absent from the majority of stations.