A constitutive description is proposed for a single pearlitic colony based on its composition of lamellas of ferrite and cementite with a very thin interlamellar spacing. Both phases are assumed to be elastoplastic. The relationship between the increments of overall stress and strain is derived and the corresponding numerical algorithm is developed. The mechanical behavior of the colony subjected to proportional or non-proportional loading is investigated, and it shows that the overall response is anisotropic. Finite element analysis is conducted to analyze the influence of the interlamellar spacing on the mechanical response of the colony. It shows that the proposed model can not only describe the behavior of the pearlitic colony with extremely small interlamellar spacing, but also work with sufficient accuracy in the case of moderate interlamellar spacing. The constitutive response of each phase in a colony can be obtained simultaneously, which is an important step towards developing theories of microstructure-based damage and failure analysis.