When recorded under Ca[2+]-free conditions, dissociated ommatidia from flies carrying two copies of Arr2^arr2 in a wildtype background show a greatly accelerated response deactivation, with a decay time constant of 94ms. In the presence of physiological Ca[2+]-containing solutions, response deactivation in these ommatidia decay more slowly than in wildtype photoreceptors. This slow decay is due to an approximately 5-fold reduction in effective quantum efficiency and an approximately two-fold reduction in quantum bump amplitude.