The colourful, vigorous territorial display behaviour of the Siamese fighting fish, "Betta splendens", has great appeal for both pet enthusiasts and animal behaviourists. Their beauty, longevity, easy maintenance and rearing make them a popular pet and an ideal science laboratory specimen. This investigation utilises "B. splendens" to test for the adaptive significance of defending a living space. Such territorial behaviour increases survival and is expected when its benefits are greater than its costs. Students can test this hypothesis by quantifying the fish's responses to intruders under different conditions in timed trials. The exercises enable college preparatory and undergraduate students in biology and psychology courses to investigate how decision-making by territorial organisms balances conflicting survival needs. Sample student laboratory results are provided to facilitate data analysis in the classroom. "B. splendens" (n = 6) displayed less to small intruders than large intruders that pose greater risk of territory takeover; and they spent less time adjacent to small intruders when highly preferred prey were available. These behavioural responses exemplify how a territorial species can reduce the costs of defence without increasing the risks of losing the territory. (Contains 1 online resource and 5 figures.)