In 1895 he was appointed professor of comparative anatomy at Sapienza University of Rome, where he would spend the rest of his life. He joined Angelo Celli, Amico Bignami, Giuseppe Bastianelli and Ettore Marchiafava, who were working on malaria in districts around Rome. Grassi was the group's entomologist. The group announced at the session of the Accademia dei Lincei on 4 December 1889 that a healthy man in a non-malarial zone had contracted tertian malaria after being bitten by an experimentally infected "Anopheles claviger". Between 1900 and 1902, Grassi, Gustavo Pittaluga and Giovanni Noè made intensive studies of malaria at Agro Portuense, at Fiumicino, on the Tiber, and on the plain of Capaccio, near Paestum.