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Estuaries are critical habitats for larvae and juveniles of many marine fishes, possibly because they promote high growth rates and survival rates. We investigated spatial and temporal changes in growth rate of larval bay anchovy (Anchoa mitchilli), in the middle Hudson River estuary where abundance of larvae is high. In two consecutive summer seasons, we sampled larvae at 4 sites evenly spaced over 45 km, at weekly intervals for up to a month. We examined otoliths to determineage in days and then used age-length regressions to estimate growth rate. In 1995, larval anchovy growth rates varied from 0.39 to 0.88 mm d-1 (median = 0.48 mm d-1). In 1996, growth rates varied from 0.41 to 0.77 mm d-1 (median = 0.55 mm d-1). In both years, we found significant spatial and temporal variation in growth rate. Larvae collected in the upper portion of Haverstraw Bay tended to grow more slowly than larvae collected in other sites. The dates on which the most rapidly growing larvae were collected varied from site to site. Neither temperature nor salinity variations explained growth rate differences. Growth rate variation, probably governed by patches of zooplankton, occurred on temporal scales of a week and spatial scales of 15 km. [ABSTRACT FROM AUTHOR]