Studies of species characteristics, number of segments and length of worm, of Helodrilus foetidus at different developmental stages (emergence from cocoon, worms without clitella and mature worms) present no evidence of continued segmentation after leaving the cocoon. There is no statistically significant difference between the means for number of segments of the intermediate and mature stages but there is a 34.5% increase in length. There is a statistically significant difference (D. of M.-r-P. E. of D. of M.=7.06) between the means for number of segments in newly hatched and intermediate stages but the increase in segments is only 5.54% whereas the increase in body length is 78.62%. Since the range in segment number of newly hatched worms is greater than in later stages (some newly hatched worms have more segments than any mature worms) it seems probable that segment number at maturity is determined by selective environmental factors rather than continued segmentation.