1. Soy-peptone has been fractionated to yield a series of increasingly purified components which sharply increase the populations of Caenorhabditis briggsae and Caenorhabditis elegans when added to the basal medium. The nutritionally active material appears to be a small polypeptide. 2. C. briggsae and C. elegans routinely reach populations of 150,000/ml or greater in 9 days in still culture, starting from an inoculum of only 500 organisms per ml. C. elegans is particularly sensitive to the depth of the medium. However, large populations can be achieved in deep cultures if continuous shaking is carried out. 3. Panagrellus silusiae shows improved populations if the basal medium is supplemented with the nutritional factor from soy-peptone. However, 0.5% acetic acid or 1% ethanol added to the medium serves equally well. There is no additive effect of ethanol and the factor.