Developmental stages of juveniles of G. rostochiensis Ro1 were studied in the susceptible cv. Marfona and in the resistant cvsAgria, Satina and Banba in glass-house conditions. The hatching response of the nematodes to root diffusates of these cvs was also investigated. Disinfected cysts of G. rostochiensis were placedin root diffusates and the numbers of hatched second stage juveniles (J₂) were counted weekly over a 6-week period. Pots were planted with a single sprout of each cv. and the soil was inoculated with a suspension of 400 freshly hatched J₂. There was a significant difference in the numbers of J₂ that had entered the roots as well as a delay of 1-2 weeks in the rate of development of juveniles in the resistant compared to the susceptible cvs. Overall, 44%, 23%, 10% and 4% of inoculated J₂ were found in the roots of susceptible Marfona, and resistant Satina, Agria and Banbacultivars, respectively. The first J₃ appeared in roots one week after J₂ entered the roots in Marfona and Satina, but after 2 weeks for the other cvs; the first J₄was found 14 and 21 days after J₂ entry in the roots of susceptible and resistant cvs respectively. The numbers of white females found on the roots were fewer than 5 or none on roots of the potatoes resistant to pathotype Ro1. When cysts were exposed to root diffusates, 74% of J₂ hatched in Marfonadiffusateas compared to 50% in Agria, 43% in Satina and 39% in Banbadiffusates.