Since the advent of gene manipulating techniques, it has become increasingly important to study the neural functional properties of the mouse. The bcl2 gene has a powerful inhibitory action on naturally occurring cell death. As a consequence the brain of bcl2 overexpressing mouse is 1.5 times bigger than the brain of a wild type animal and the retina has more than twice the ganglion cells than normal (Martinou, Dubois-Dauphin, Staple, Rodriguez, Frankowski, Missotten, Albertini, Talabot, Catsicas, Pietra, & Huarte (1994). Neuron, 13: 1017-1030). Since in most mammals the upper limit of behavioural visual acuity is imposed by ganglion cells density, the visual acuity should be higher in bcl2 mice than in wild type mice. We measured behavioural visual acuity in wild type and transgenic mice and, contrary to the expectation, we found it to be of the same order (0.5-0.6 c/deg) in the two groups of animals, indicating that an increase in ganglion cells density is not effective in improving visual resolution.