Research was conducted in pasture, cerrado forest fragments, and crop succession areas with the objective of studying the biology and describing the third instar of Bothynus medon (Germar, 1824). The studies were conducted in Aquidauana, Mato Grosso do Sul and Tangará da Serra, Mato Grosso, Brazil, where adults were captured using light traps baited with ripe banana and sugarcane juice; larvae were sampled in soybean and corn crop areas. Adults were encountered flying in the field at the start of the rainy season, which lasted from September to December. Eggs were found in October and November, first and second instars occurred from November to February, and third instar from January to July. Pupae were encountered from July to August. Development from egg to adult was completed in a year. A description of the third instar of B. medon and a key to known larvae of the genera of New World Pentodontini are herein presented.