Many livestock sector models have limited coverage of relevant variables, and are somewhat ad hoc in their choice of what should be specified as behavioral equations. This study develops a generic conceptual approach to modeling the livestock sector that provides consistent rules of specification and better coverage of variables. This approach is then applied to the swine-pork sector of Japan. The new approach departs significantly from existing models. The structure clearly differentiates stock and flow variables; only flow variables have behavioral specifications and stock variables are accounting identities; flow variables are expressed in rates rather than levels; logistic functions are used in most flow variables to automatically impose biological-technological limits; and swine slaughter number and weight are disaggregated into sow and other swine (i.e., gilt-barrow). The estimated Japanese swine-pork model has good fit, no serial correlation, significant coefficients, correct signs, and is better able to capture both the mean and variability of all endogenous variables.