Grassé continued his studies in Paris, focusing exclusively on science. He obtained his Licence in Biology and frequented the laboratory of biologist Étienne Rabaud (1868–1956). He abandoned his preparations for the agrégation to accept a position as professor in the École Nationale Supérieure Agronomique de Montpellier (1921), where the department of zoology was led by François Picard (1879–1939). There he frequented several phytogeographers like Charles Flahault (1852–1935), Josias Braun-Blanquet (1884–1980), Georges Kuhnholtz-Lordat (1888–1965) and Marie Louis Emberger (1897–1969). He became the assistant of Octave Duboscq (1868–1943) who oriented the young Grassé toward the study of protozoan parasites. After the departure of Duboscq to Paris, Grassé worked for Eugène Bataillon (1864–1953) and there discovered techniques for experimental embryology.