Many laboratories have solely used the Wilson isolate to physiologically characterize the harmful algal bloom (HAB) dinoflagellate Karenia brevis (C. C. Davis) G. Hansen et Moestrup. However, analysis of one isolate may lead to misinterpretations when extrapolating measurements to field populations. In this study, pulse-amplitude-modulated chlorophyll fluorometer (PAM-FL) relative electron transport rate (ETR), F v ⁄ F m , and chl were compared with traditional techniques, such as 14 C photosynthesis versus irradiance (P-E) curves, DCMU [3-(3¢,4¢-dichlorophenyl)-1,1-dimethyl urea] F v ⁄ F m , and extracted chl. The DCMU and PAM-FL values of F v ⁄ F m (r 2 = 0.51) and chl (r 2 = 0.58) were in good agreement. There was no correlation between 14 C and PAM-FL a, P max , and b parameters because PAM-FL ETR was only a relative measurement. The PAM-FL techniques were then used to investigate P-E curves, quantum yield of PSII (F v ⁄ F m ), and chl from 10 K. brevis isolates to determine whether one or all isolates would better represent the species. Comparisons were made with a radial photosynthetron, which allowed for controlled conditions of light and temperature. Isolate a, P max , and b varied between 0.097 and 0.204 lmol e ) AE m )2 AE s )1 AE (lmolquanta AE m )2 AE s )1 ) )1 , 80.41 and 241 lmol e ) AE m )2 AE s )1 , and 0.005 and 0.160 lmol e ) AE m )2 AE s )1 AE (lmol quanta AE m )2 AE s )1 ) )1 , respectively. Either carbon limitation and ⁄ or bacterial negative feedback were implicated as the cause of the P-E parameter variability. Furthermore, these results directly contradicted some literature suggestions that K. brevis is a low-lightadapted dinoflagellate. Results showed that K. brevis was more than capable of utilizing and surviving in light conditions that may be present on cloudless days off Florida.