Behavioral and event-related potential (ERP) measures were used to assess cognitive workload from expert computer gamers playing a "first-person shooter" video game. Game difficulty level was manipulated in separate conditions by adjusting the number of enemies (view, easy, medium, and hard). Infrequently presented single-stimulus tones were either ignored or counted across difficulty conditions. Game performance and tone-counting accuracy declined as game difficulty increased. ERP component amplitudes diminished for both the tone ignore and counting conditions as game difficulty increased. The findings suggest that cognitive workload induced by video gaming can be reliably assessed through behavioral and neuroelectric means, and that the single-stimulus paradigm can be a useful tool for evaluating workload in an immersive stimulus environment with less distraction than conventional tools.