This paper address a special class of generalized assembly line balancing in which it is assumed that there are two groups of workers: skilled and unskilled ones. The skilled workers are hired permanently while the unskilled ones can be hired temporarily in order to meet the seasonal demands. It is also assumed that more than one worker may be assigned to each workstation. To show the advantages of assigning several workers instead of single workers to each workstation in such a class of problem, a mixed integer programming formulation is presented. This model minimizes the number of temporary workers on the line as the first objective and the number of workstations as the secondary one while cycle time and the number of permanent workers are fixed. The proposed formulation is applied to solve some experimental instances found in the literature. The comparison between the optimal solutions of the proposed model and those of traditional assembly lines with a single-manned workstation indicates that our model has been able to reduce the line length on average of 24.40 per cent while the number of unskilled workers remains optimal.
