An important center in which genetic research started and was carried out in Brazil during the 20th century was situated at the Faculty of Philosophy, Sciences and Linguistics of the University of São Paulo, led by André Dreyfus (1897–1952). Beginning in 1943, the Ukrainian geneticist Theodosius Dobzhansky (1900–1975) visited Dreyfus’s group four times. This paper evaluates the impact of Dobzhansky’s visits on the studies of genetics and evolution developed by the members of Dreyfus’s group during the 1940s and the 1950s. The study leads to the conclusion that Dobzhansky’s visits had an impact, not only in quantitative terms (the number of individual and joint publications), but also in qualitative terms. However, we also detect a decrease in the number of individual and joint publications related to the subject of the project during certain periods. The adoption of new experimental organisms by some members of the group; the involvement with subjects not related to the initial project, such as botany; Dobzhansky’s and his wife’s health problems during the third visit; and scientific disagreements between Dobzhansky and Brazilian researchers may have contributed to the decrease in publications.