The oxygen-evolving properties of broken chloroplasts isolated from biotypes of Chemopodium album and Amaranthus retroflexus that were either sensitive or resistant to s-triazine herbicides were compared. The pattern of oxygen flash yields produced by herbicide-sensitive, dark-adapted chloroplasts of either species was reminiscent of that found with spinach chloroplasts. In contrast, dark-adapted chloroplasts isolated from the herbicide-resistant biotypes exhibited a highly damped oxygen flash pattern in which there was significant oxygen released after the first light flash. Analysis of these results with the Kok model of photosystem II (Kok, B., Forbush, B., and McGloin, M. 1970. Photochem. Photobiol. 11: 457–475) suggested that the unusual properties of the resistant organelles were due to the survival of significant amounts of S3 and S2 states during dark adaptation and to a higher proportion of inactive photosystem II reaction centres during each light flash. Deactivation experiments verified the suggestion that the S3 and S2 states more readily survive a 10-min dark period in resistant organelles. Information about electron transport on the oxidizing side of photosystem II was obtained with a modulated oxygen electrode and suggested that there was no difference between the two biotypes in the value of the rate constant of the reaction that limits the rate of electron transport between the water-splitting step and photosystem II.