Observations as to the number of pre-adult instars have been made on over 200 individually reared female Daphnia magna of seven clones. In all cases measurements of the total length during each instar were taken. For well over a hundred individuals measurements of carapace length and height were taken in addition to those of total length. The number of pre-adult instars varied upward from five. This variation was found in all clones tested. Growth curves have been constructed for various groups based on the number of pre-adult instars. The inflection in the growth curve of any dimension in any group coincides with the time of sexual maturity. "Brooks' Law" holds only for those groups which were primiparous during the sixth instar and then only approximately. Relative growth in the dimensions studied, x and y, may be expressed by the equation [See equation in the PDF file] Relative growth changes at sexual maturity, i.e., a change occurs in the values of the constants b and k. The above equation may perhaps be used to advantage in the study of variation in other Cladocera. By the use of this equation the direction of variation can be given numerical values. Comparisons of different races and varieties, both seasonal and geographic, may be readily made on this basis.