A highly prevalent type of mosaic that induces heavey losses to sweet pepper (Capsicum annuum L.) plantings in São Paulo and other neighboring states is caused by two groups of the potato virus Y complex, designated as n and w. Group n is represented by strain Yn; group w by strains Yw and Yf. Screening tests with 45 sweet pepper varieties and 46 others of the pungent type, belonging to several species of Capsicum, were carried out in the greenhouse and in the field to detect resistance to mosaic. A hypersensitive type of resistance to Yn was found in the sweet pepper varieties Mogi-das-Cruzes and Casca-Dura. Porto Rico Wonder was tolerant to Yw, but not so to a related strain, Yf. A sweet pepper variety called Moura was highly tolerant to Yf. A variety known as P 11 (P.I. 264281) (Capsicum annuum) was found to be immune to all Y virus strains tested. The pungent types SA 112 (C. pubescens), 1-30771 and 1-30772 (C. pendulum) also behaved immune to Y virus strains, but were not used as sources of resistance because they are not compatible with C. annuum. Inheritance studies disclosed the existence of 3 new pairs of genes responsible for the various types of resistant reactions to strains of the potato Y virus in Capsicum annuum. They can be described as follows: Gene "H" is a dominant allele found in Mogi-das-Cruzes and Casca-Dura, that conditions hypersensitivity to Yn. Gene "w" is a recessive factor that controls tolerance of Porto Rico Wonder to strains of Yw. Gene "f" is a recessive factor that conditions tolerance to Yf in the sweet pepper variety Moura. Gene "f" behaves as an allele of "w", affording a higher level of tolerance to strains of the Y virus complex. The immunity to various strains of the Y virus complex found in Florida in P 11 (P.I. 264281) by COOK & ANDERSON (7), and attributed by these investigators to a pair of recessive genes "yªyª", was also verified for strains of the Y virus complex present in São Paulo. The sweet pepper varieties Yolo Wonder and All Big were introduced in the breeding program because they carry the partially dominant genes "LiLi" (17) that controls resistance to tobacco mosaic virus in pepper. Early in the development of the present breeding program, the genes "HH", "ww" and "LiLi" were combined in several improved lines, but appearence of a new strain Yf showed the need of a higher resistance level than that present in these tri-hybrid lines. The incorporation of the Moura type of tolerance due to genes "ff" permitted the development of commercial lines highly tolerant to strains of the Y virus complex. The immunity to strains of the Y virus complex from P 11, due to genes "yªyª", was incorporated to sweet pepper genotypes already possessing the other types of resistance to Y mosaic and to TMV. This resulted in two new groups of sweet pepper varieties that are immune to all strains of the Y complex so far tested, resistant to TMV, high yielding, and with good commercial characteristics: Agronomico 7 has fruits of the blocky type (California Wonder type) and Agronomico 8 has tapering, elongated fruits (Casca-Dura type).