A population of N. diversicolor was studied over a period of one year, at Chalkwell, Essex, England. It was concluded that in this population, length of life did not normally exceed 18 months. The population was replaced by a single spawning in February, young worms growing 1.0-2.0 cm. each month, and reaching sexual maturity in 12 months. Worms die after spawning, but owing to the sexual behavior and rarity of males, large numbers of females remain unspawned, surviving another 6 months and some attaining a relatively large size. Graphs and tables showing the rate of growth, mean length, and population density throughout the year, are given.