SUMMARYChanges in populations ofHeterodera goettingianaLiebs. were studied in microplots containing three kinds of soil. Starting with an artificially uniform population, varieties of pea which matured quickly induced smaller numbers of cysts and eggs than later‐maturing varieties although there was ample time for completion of the life cycle on the former. Field bean was comparable with the late‐, and broad bean with the early‐maturing pea varieties.When one variety of pea was grown on all plots of a group, yield decreased in proportion to the density of the population before planting. The relationship between yield and initial population over a narrow population range was expressed equally well by the simple equationsY=a+bxorY=a+blogx, whereY= yield,x= the number of eggs,a= theYintercept andbthe regression coefficient. For plots with a wide range of initial populations (4–359 eggs/g. soil), neither equation was adequate and the relationship was best fitted by a sigmoid curve.In two sets of plots, the influence of two population levels, nitrogen versus no nitrogen, and light versus heavy soil, was tested. Soils and populations affected yield significantly; nitrogen was ineffective on light soil but increased yield on the heavy soil. Only soil type affected the final population, that in the heavy soil which gave the greater yield being twice as large.When peas were grown, the final population rose to a ceiling level independent of the initial population size. In the plots with widest initial population range, the ceiling level was determined by the number of females (new cysts) formed. Their number was almost constant over the whole initial population range and they produced on average 129 eggs/cyst which was the maximum theoretically possible rate of increase under the conditions of the experiment. At small initial egg densities (4 eggs/g.) about 50% of the eggs became female and at great densities (359 eggs/g.) only 1% or less.Final populations in the microplots bore no relationship to yield of peas. It is suggested that the size and form of the central part of the root system determines how many females the roots can support and that root size is one of the factors that determines ceiling levels.Nematicides used to decrease numbers of cyst‐nematode eggs or larvae in soil tend to remove only those surplus to the carrying capacity of root systems. Nematicides or enemies able to kill established females before they produce eggs would control reproduction more ef