This study investigates the significant presence and function of nonhuman elements, specifically flora and fauna, in Aluísio Azevedo’s seminal Brazilian naturalist novel, O Cortiço (1890). Drawing on the increasing academic interest in plant and animal studies in literary criticism, this analysis catalogs and categorizes the numerous references to plants and animals, as well as instances of animalization, to illuminate Azevedo’s naturalistic portrayal of the urban environment of Rio de Janeiro. The research demonstrates how, in line with naturalist principles, Azevedo employs these nonhuman comparisons to characterize his human figures, often reducing them to their physical or instinctual traits under the deterministic influence of the milieu. The study investigates patterns in the use of flora and fauna where both are frequently used to evoke sensuality, purity, the physical states of characters—often reinforcing social hierarchies, reflecting racist and patriarchal views. Ultimately, this study argues that Azevedo’s extensive use of flora and fauna in O Cortiço is crucial to conveying to naturalist ideas, characterized by degeneration, decay, and the leveling of distinctions. The constant interplay of the characters and their environment, mediated through plant and animal allegories, underscores the deterministic forces at play, where individuals are subject to the relentless and often brutal influence of heredity and their surroundings. This analysis contributes to a deeper understanding of Brazilian naturalism and the sophisticated ways nonhuman elements can shape and influence narrative meaning.