Improvement of the São Paulo cotton varieties origin and breeding of IAC 13-1The origin and breending of the São Paulo cotton variety IAC 13-1, as well as the involved proceedings and numerical results are reported. The "eastern Acala" Acala 5675, selected at Knoxville, Tenn., from a Mexican cotton was introduced to São Paulo in 1950. It showed very good fiber qualities in the new environment. After nine years of acclimation and individual selection it yielded a promising line named IAG 61/60. IAC 13-1 was developped from this line by the general pedegree selection process adopted in breeding for final genetic balance by the Cotton Section of the Instituto Agronômico. The first stock of genetic seed was released for further increase in 1968. IAC 13-1 substituted the commercial variety IAC 12-2. This substitution signified an improvement in yield, type and size of bolls, size of seeds, fiber qualities and behaviour in areas contaminated by Fusarium oxysporum f. vasinfectum (Atk.) Snyder et Hansen.