An investigation into the biology of the potato cyst nematodes (PCN) Globodera rostochlensis (Ro) and G. pallida (Pa) was undertaken in the South Island of New Zealand. The study followed two basic pathways. (1) An examination of their ecology in order to identify factors which influenced their population biology and effect on the potato host. (2) A comparative study of the two species in the same geographical area where they were subjected to the same environmental factors. The two species were studied In isolation and in mixed cultures. Potato cyst nematodes establish well in the South Island and multiply quickly under repeated annual cropping of the host. In replicated field microplots with low pre-plant densities a maximum multiplication rate of about 90x was observed. A mean maximum density of 300 eggs/ml of soil was measured and an equilibrium density of about 150 eggs/ml was achieved with continuous cropping. Significant yield loss of host crop occurred at a pre-plant density of 15-20 eggs/ml of soil and the percentage crop loss increased at higher egg densities untiI maximum loss of 95% was obtained. No difference in the pathogenicity of the two species was detected. Nematodes in early cropping ground at Outram had a much lower multiplication rate which varied through the season. Early harvesting of crops could reduce the multiplication rate substantially. If the crop was allowed to mature, multiplication rate rose by up to six times. The cyst population produced from early harvested crops could be divided into three components, (i) cysts produced early that fell off the roots at harvest and matured in the soil, (ii) cysts produced later that were still imbedded in roots as immature females at the time of harvest and developed to maturity on the moribund root and (iii) residual eggs which occurred within the cysts of the initial inoculum. Early maturing cysts made the greatest contribution to the inocuIum potential for the following crops whereas residual eggs made the least. The presence of selfset potato tubers had differing influences on the three inoculums components. Life tables were constructed for both species at three locations (soil types) and for two planting dates (soil temperature differences). Larvae within the cyst (life style A) had low mortality but once hatched and free in the soil (life style B) mortality increased. Soil temperature affected the level of mortality. Once established in root tissues (life cycle C) mortality decreased but rose slightly when adult females matured (life style D) and protruded into the soil. Soil temperature and host vigour influenced the fecundity of developing females. Comparisons of the two species in the same environment showed that Ro hatched earlier, and suffered less mortality and had a shorter duration than Pa in life style B. Once inside the root tissue (life style C) mortality of Ro was often greater than in Pa. Life styles C and D were also of shorter duration in Ro than Pa under most conditions. When mixtures of the two species were kept together on the same host Ro was competitively superior and was proportionately more dominant in the next generation. This was observed over a range of Ro:Pa ratios. Reduction in overall nematode density appeared to improve the relative success of Pa. At very low proportions Pa is able to maintain its position and an equilibrium between the two species is established. Spontaneous hatching (the major component of population attrition) in the absence of host stimulation occurred in both species and followed an exponential decline curve over time. Soil type influenced the attrition rates which were highest in Canterbury silts. Attrition rates differed significantly between species and were higher in Ro. Cyst size had Iittle effect on attrition rate. Reduction in egg numbers observed with time was largely a result of active egg hatching rather than within cyst mortality. This study supplied the basic biological information on potato cyst nematode In the South Island of New Zealand, and when combined with the results of chemical control experiments and advances in the breeding of resistant potato cultivars has produced a largely successful control and management program for this pest.