The growth and nutrition of Ectocarpus confervoides has been studied in bacteria-free cultures. Growth was estimated by dry weights determined under controlled conditions. A method for inoculation of cultures of filamentous algae is described and a table of culture media is given. Following a lag of 5–7 days there was a period of growth, which was not truly exponential, extending over a period of 35 days. Calculation of the relative growth constant for the early stages of growth indicated that the alga doubled its dry weight every 6–7 days. Cultures of Ectocarpus remained viable for over 1 year in the light and for over 100 days in the dark. In natural sea-water media the maximum growth was brought about by the addition of 0·5 mM potassium nitrate and 0·1 mM potassium phosphate. Ferric chloride had no effect on growth but additions of manganese chloride did cause some stimulation. Additions of some complex organic mixtures slightly increased growth but Ectocarpus was unable to grow on a range of organic carbon source in the dark. The optimum salinity at 20° C was somewhat higher than that of natural sea water but was apparently lower at 15° C. The optimum pH was 8·0. Light intensities between 1350 and 16,000 lux had no marked effects on the growth rate but did effect the lag, the optimum for this being 7000 lux. Temperature also had no effect on the growth rate but did effect the lag. The optimum was 15–20° C but there was growth between 10 and 25° C. A considerable range of artificial sea waters, with and without organic additions was investigated, in no case did they give a yield greater than two-thirds that in natural seawater media. These results are discussed in relation to other workers findings.