Photosystem II (PSII) inhibitors are ubiquitous in costal areas around the world. Changes in chlorophyll fluorescence have been seen in plants exposed to low concentrations of PSII inhibitors in laboratory experiments. Saw wrack (Fucus serratus), bladder wrack (Fucus vesiculosus) and Fucus evanescence are important and widespread in Norwegian coastal ecosystems, as primary producers and as structure forming perennial species. Ability to sensitive and rapidly monitor adverse effects in these species could be useful in a monitoring program. Two chlorophyll fluorescence parameters, photosynthetic efficiency (how efficient quanta is used in PSII) and quinone pool (amount of electron acceptors in PSII) were measured by Hansatech Handy Photosynthetic Efficiency Analyser. Few differences in these parameters were seen in F. vesiculosus was grown under different irradiances, temperatures and salinities in three laboratory studies. Photosynthetic efficiency and quinone pool in F. serratus and F. vesiculosus were adversely affected by 100 µg L-1 Irgarol, but not by the other biocides tested. In a field survey at 6 sites in the Oslofjord from April to December 2007, differences were seen between fluorescence parameters in samples from different sites. Photosynthetic efficiency and quinone pool were highly correlated with light intensity, temperature and salinity during the survey, but this could not explain the difference observed on all days. Analysis of Irgarol in Fucus tissue from different sites did not either clarify the observed differences. Measurement of photosynthetic efficiency and quinone pool in Fucus spp. is discussed in relation to ecological relevance and other biomarker methods.