An unstructured model based on mass balance equations for a recombinant methylotrophic yeast Pichia pastoris Mut(S) (Methanol Utilization Slow) strain expressing the mini-proinsulin (MPI), was successfully established in quasi-steady state fed-batch fermentations with varying total quantity of biomass in a 7 l fermenter. The model describes the relationships between the total biomass and induction time, both in the batch and fed-batch phases. In addition, good correlations were obtained when the total quantity of MPI was correlated with the total biomass, demonstrating that the product of interest is associated with growth in the methanol phase. The parameters of the fermentation model obtained are similar to those reported by other researchers.