The infectivity and development of two biotypes of citrus nematode (Tylenchulus semipenetrans) were compared on highly resistant Poncirus trifoliata selection 'Pomeroy,' moderately susceptible 'Troyer' citrange, and highly susceptible sweet orange selection 'Homosassa' small seedlings in a glasshouse. Biotype-1 was more infective on the above hosts and developed faster on sweet orange and on 'Troyer' citrange than Biotype-2. The differences in infectivity were interpreted to reflect differences in the ability of the nematodes to penetrate the epidermis and hypodermis and become established in host roots. Poncirus selections 'Pomeroy,' 'Webber-Fawcett,' and 'Rubidoux' seedlings were highly resistant to the citrus nematode in California, but seedlings of 'Pomeroy' and 'Rubidoux' were only moderately resistant in Japan. These differences in degree of infection may indicate different biotypes of the nematode. Host range tests with California Biotype-1 indicate that it differs from those occurring in Israel.