Variation in susceptibility to infection with cultured promastigotes of Leishmania major was detected among 3 different geographic strains (ISRAEL, EGYPT, and INDIA) of the sand fly Phlebotomus papatasi. The ISRAEL strain showed the greatest susceptibility and was chosen for subsequent genetic selection experiments. After 13 generations of genetic selection in this strain, a stable refractory line was obtained in which only 7.5% of the insects could be infected. A highly susceptible line was also obtained in 5 generations of selection. Infection rates in the susceptible line were stabilized at about 94%. A description of the selection process and characteristics of the 2 lines are given.