Microscopic, brown algal crusts and filaments were collected in the summer from an oyster reef on the Atlantic coast of Virginia and isolated into culture. Developmental studies showed that the isolates were microscopic stages ofPetalonia andScytosiphon. These two brown algae exist the year around in the form of either microscopic or macroscopic plants. In culture no sexual reproduction was found linking the two stages of eitherPetalonia orScytosiphon. The zoospores from the macroscopic plants ofPetalonia andScytosiphon developed directly into crusts and initials on the crusts developed directly into macroscopic plants. Culture conditions of 10 C and a 9–15 hour photoperiod stimulated the development of macrothalli in both taxa.