The population dynamics of Hypera postica (Cyllen- hal) eggs was studied at 2 northern and 2 southern locations in Ohio during ]965-68. Weekly or bi-weekly square-foot samples of plant material to ground surface were collected from October to June and dissected by hand to determine the egg population. Samples of no less than 58% of the eggs found were incubated and examined for viability and incidence of parasitism by Patasson luna (Girault) (Hymenoptera: Mymaridae). Populations of eggs during late fall and winter were generally high at the southern locations and low at the northern locations. Maximum populations occurred during the 2nd to 3rd week of May at the northern locations and the last week of April to the 1st week of May at the southern locations. Most eggs in the spring were found in miscellaneous dead stems until new alfalfa Stems were at least 10 in. tall. Following this, samples from the northern locations showed the number of eggs in green alfalfa increased rapidly to 85-95% of those found. However, samples from the southern locations indicated that 50% were still being laid in miscellaneous dead stems. Cluster size ranged from 3 to 19 eggs per cluster with an average of 9.6 eggs per cluster occurring in green alfalfa stems and 9.3 eggs per cluster in miscellaneous dead stems. Plants, other than alfalfa, in which eggs were found included: red clover, Trifolium pratense (L.); hen bit, Lamium amplexicaule (L.); chickweed, Stellaria media (L.) Cyrill, and miscellaneous grasses. The mean egg viability for all locations from October to December was 89%. During 1966 and ]968 viability from February to mid March was reduced to 0-26% at the northern locations. Mean viability for all locations from late March to June was 83%. Parasitism by P. luna was rare from October to February and generally common from March to June. In general, the mean percent parasitism from March to June was 1.9% and the range 0-16.4%.