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The response of plastic scintillators to the flux of particles occurring in cosmic ray air showers has been measured at various distances from the center of showers of various sizes. The number of ``scintillator particles'' S obtained by dividing the observed pulse height by the mode of the pulse‐height distribution due to single fast muons has been compared with the number of incident ionizing particles G seen by Wilson cloud chambers and Geiger counters. The ratio S/G is always greater than unity. It increases with increasing number of charged particles per square meter, being 1.1 for 100 scintillator particles per m2 and 4.0 for 10 000 scintillator particles per m2. It varies with radial distance from the shower center and with shower size, being 1.2 on average at 30 m from the center of showers of ∼105 particles and greater than 4 near the center of large showers. The increase in S/G is partly due to cascading of the electromagnetic component within the plastic scintillator but mostly to nuclear interactions produced in the scintillator by strongly interacting particles.