The morphological ontogeny of Hydrozetes paulista Pérez-Íñigo & Baggio, 1989 is described and illustrated. This species is monodactylous as adult, with short, setiform bothridial seta and belongs to the “lacustris species group”. It has 13 pairs of notogastral setae, including c₂, but seta c₃ is retained by some individuals. Most gastronotal setae of larva are of medium size and barbed, except for longer setae lm, lp and h₂, and short and smooth c₁ and h₃. In the nymphs setae of l-series, h₁ and h₂ are very long, smooth and dark pigmented, setae h₃ and p₁ are of medium size, thick, pointed and barbed, and other setae are short and usually smooth; seta lm is located anterior to the gland opening. Hydrozetes paulista is bisexual, but in this study only females were present.