Mated females of the western tarnished plant bug, Lygus hesperus Knight, are distinguished from unmated females by the presence of one or more spermatophores. The presence of a spermatophore is normally determined by dissection. A simple and nondestructive method to distinguish mated L. hesperus females from unmated females would facilitate laboratory studies that require mated insects. Spermatophores are visible through the abdominal sternites of recently mated L. hesperus females, but the consistency and persistence with which spermatophores are externally visible has not been previously documented. The overall objective of this study was to evaluate whether examination for the presence of externally visible spermatophores is a reliable method to determine whether L. hesperus females have mated. The presence of externally visible spermatophores correctly discerned 99% of recently mated (≤24 h) females from unmated females. None of the unmated females were misclassified as mated. The apparency of visible spermatophores decreased with increasing time after mating until the spermatophores were no longer visible. The period during which spermatophores were externally visible decreased linearly with increasing temperature from 15.6 - 29.4°C. Females continued to oviposit fertile eggs after spermatophores were no longer externally visible. Results indicate that examination for the presence of externally visible spermatophores is a reliable method to discern mated female L. hesperus from unmated females in controlled laboratory studies. Because spermatophores become less apparent with increasing time after mating, this method is not suitable for the classification of mating states of field-collected insects or others of unknown history.