This thesis investigated the behaviour and physiology of the visual system of Sepia officinalis by studying systematically its visual sensitivity and its spatial resolution. The cephalopod retina is composed of only photoreceptors and supporting cells, thereby providing a unique opportunity to study the interactions between photoreceptors, without the influence of other neurons, such as those typically found in the vertebrate retina. The minimum separable angle (MSA), a measure of spatial resolution, of S. officinalis was determined from behavioural experiments to be 42' for animals of 8 cm mantle length at 15 µW/cm² light intensity. As the animals grew in size and as the ambient light intensity was increased, S. officinalis showed improved visual acuity. Through these experiments, it was revealed that each tested size of animal adapted to light with similar efficiencies, and that factors other than retinal growth were involved in the improvement of behavioural MSA with increasing size. The minimum light intensity to which the retina of S. officinalis responded was 0.1 µW /cm2 , which was slightly higher than that to which individual photoreceptor cells responded. Retinal sensitivity decreased with increasing animal size. This was unexpected, as previous theoretical and behavioural studies in other species have shown sensitivity to increase with increasing animal size. Possible reasons for the decrease in sensitivity were a reciprocal decrease in cell resistance or an increase in dark noise. The visual sensitivity of S. officinalis was also affected by the stimulus flash wavelength and duration. Its retina adapted to background light in a way similar to vertebrate photoreceptors and the extracellular calcium concentration of the solution perfusing the retina affected this process. Finally, two series of experiments provided some evidence that functioning gap junctions exist in the retina of S. officinalis. By completing a study of the visual sensitivity of S. officinalis at the three levels of single cell, retina and whole animal, the visual processing that occurs between these physiological levels was investigated. From the work presented in this thesis, it is concluded that, although S. officinalis did not prove comparable in every aspect to other species on an intracellular level, it would be a useful model of behavioural and extracellular visual processes for both invertebrate and vertebrate species.