Size fractionation has been used to isolate L1/L2 larvae from mixed cultures of the nematode, Caenorhabditis elegans. Worm lengths have been compared during growth in synchronized liquid and agar cultures. Supplementation of liquid S medium with 10 ppm surfactant (Pluronics F-68, F-127, F-38, L-35; Tween-20 or Triton X-100) promoted a significant stimulation of growth over three clays in all cases. Because of possible poor nutrition and/or aeration in liquid culture, experiments were repeated using the same surfactants in standard NGM agar. Four surfactants again stimulated worm growth significantly whereas two (Tween-20 and Pluronic F-68) did not. Differences between these surfactants were also demonstrated with respect to : (i) toxicity; (ii) induction of stress responses in a transgenic hsp/reporter strain; and (iii) stimulation of amino acid incorporation into soluble protein both initially and after 41 h of surfactant treatment. These surfactants, and in particular, Pluronic F-127, are potentially valuable as culture supplements for enhancing nematode larval growth. Possible mechanisms for growth promotion by surfactants are discussed in light of the fixed somatic cell lineage and the fact that Pluronic F-127 did not speed up maturation from L4 larvae into egg-bearing adults.