Looming objects elicit protective behaviors in both humans and animals. For example, frogs exhibit escape behavior (which is a protective behavior) in response to encountering looming objects. Previous studies have demonstrated that synchronized oscillatory activities among dimming detectors (class-4 neurons) encode essential information for escape behavior in frogs. However, oscillatory retinal output is only one of the several necessary conditions required to elicit escape behavior. Therefore, in the present study, both electrophysiological and behavioral experiments were performed to examine the neural representation for looming stimuli in frog retinas. The neural activity in multiple ganglion cells was also recorded with a planar multi electrode array (MEA). To identify stimulus features essential to eliciting escape behavior, various visual stimuli were systematically presented to frogs. The results of the behavioral experiments revealed that the expansion of the solid-dark area was necessary to elicit escape behavior. To explore how expansion was coded by the retinal ganglion cells, we investigated the firing rate of retinal ganglion cells in response to an expanding dark square or other motion stimuli. Among the four types of ganglion cells that were recorded, the class-3 neurons showed the most expansion selectivity. Correlation analyses between the firing rates of retinal ganglion cells and escape rates revealed that class-1 and 2 neurons may not play functional roles in eliciting escape behavior. It had already been demonstrated that synchronized oscillatory activities of class-4 neurons correlated with both stimulus size and continuity. Therefore, the firing rate of class-3 neurons might encode expansion of looming objects, and the synchronized oscillatory activities of class-4 neurons might encode both solidity and size of looming objects.