We have studied the synergistic action of deltamethrin and prochloraz in bees in laboratory experiments that allowed us to express dosages in terms of field rates (g ha−1). It was established that, used alone, deltamethrin at 0·125 g ha−1 and prochloraz at 25 g ha−1 did not produce mortalities different from that of the control during 96 h of observation. Sprayed as a mixture of these doses, deltamethrin and prochloraz produced 67·5% corrected mortality within 24 h and 74·1% corrected mortality within 50 h. Sequential treatments of deltamethrin and prochloraz spaced by a 0·8-day interval reduced the synergistic action of both molecules. At 50 h, the lethal effects were 27·5% corrected mortality for the treatment of deltamethrin followed by prochloraz and 23·8% corrected mortality for the treatment of prochloraz followed by deltamethrin. Results are discussed in terms of mode of action and sub-lethal effects.