Regression equations are provided for the earthworm Eisenia foetida with respect to age at which 50% of the population became clitellate at 25° C in relation to population density in activated sludge and in horse manure. Regression equations are provided for progeny per cocoon versus weight of cocoon, and weight of cocoon in relation to weight of parent; from these an equation is derived for progeny per cocoon relative to worm weight. Regression equations are given on (a) number of cocoons produced per adult in relation to age and population density from onset of adulthood to median peak production of cocoons, age 10 weeks, and from age 10 weeks to age 27 weeks, and (b) weight of worm in relation to population density and age between ages 5 and 27 weeks. From (a) and (b) a family of equations (c) are derived giving progeny per cocoon in relation to age of adult and population density. From equations (a) and (c) two families of equations are generated giving progeny per adult in relation to ascent to, and descent from, the median week of peak cocoon production in relation to population density. Data also are provided on age at which reproduction terminates in relation to population density, optimum population density for reproduction, and hatchability.