In an assembly system with a fixed number of workers, job enlargement leads to parallel lines. Job enlargement reduces absenteeism and turnovers, thereby increasing productivity. Job enlargement also reduces the number of repetitions per period reducing the learning effect and hence productivity. In an optimal job design, the loss of learning must be traded off with the reduction in absenteeism and turnovers. In this paper we show that an optimal job design exists with respect to the system response time and propose an analytical model to achieve such a design. Our experience with the model suggests that an optimal job design is most important when a new system is considered, and when significant learning takes place.