Total phosphorus concentrations were determined in rainwater collected at the Bayside Laboratory, Lewes, Delaware from February 1966 through January 1967. The mean concentration of winter-spring rainwater was 4.9 μg A/liter, that of summer rainwater increased to 150 μg A/liter, and fall-winter levels decreased to 8.1 μg A/liter. This phenomenon may account for atypical phosphorus cycles noted in estuarine areas.