The yields of Italian and hybrid ryegrass cvs Promenade and Inca respectively and of tall fescue cv. Pastelle were decreased by M. naasi for the three first months of their growth. The second stage juvenile populations within the roots during establishment were responsible in 1984 for losses of seedlings of Inca and Promenade. Negative correlations were statistically significant between these root populations and the height of both varieties, but dry and fresh plant weight was diminished only in cv. Promenade. The initial population in the soil (Pi) was significantly negatively correlated with dry weights of the plants of all three cultivars after three months growth. At the time of the second yield assessment cvs Inca and Pastelle were tolerant to M. naasi, by contrast with cv. Promenade. The study of the relationships relating to Pi and the root populations of the nematode revealed that J2 invaded the roots of grasses independently of the Pi, but three months after sowing, the number invading was proportional to the Pi. The increase of J2 populations in the roots may have been the consequence of invasion throughout the experiment only at the highest levels of infestation. The development of J2 into females in cvs Inca and Pastelle was negatively related with infestation level, but was independent of Pi in cv. Promenade.