A detailed model of the diel vertical migration of euphausiids, particularly that of Meganyctiphanes norvegica adults in the northwestern Mediterranean Sea, is presented. It simulates the temporal and vertical variation of the euphausiid biomass over the diel cycle in the water column from 0 to 700 m depth. Migration is influenced by two external factors: food (phyto- and zooplankton) and light. Light is assumed to influence migration both by its intensity with the occurrence of an optimal value, and by its relative change in intensity, which affects the ascent or descent of the organisms. The general features of the diel vertical migration of adult euphausiids are reproduced by the model which simulates day-time depths of 256-416 m, night-time depths of 8-64 m, migration amplitudes of 208-372 m and an ascent and descent migration taking 3 hours. In order to define further laboratory studies and field samplings, tests of the sensitivity of the model have been performed by varying the values of some of the parameters, changing the mathematical expression of some of the processes and using different food distributions. In this way, we see the effects of the assumptions on timing and amplitude of the migration.