Lyngbya majuscula is a filamentous marine cyanobacterium with a worldwide distribution in temperate and tropical regions to a depth of 30m. Over 70 chemicals have been isolated and characterised from this organism, many of which are biologically active. Dramatic responses have been elicited after human exposure to lyngbyatoxin A (LA) and debromoaplysiatoxin (DAT), toxins extracted from L. majuscula. These chemicals have been found to cause irritation at concentrations as low as 100 pmol and exposure of humans to this cyanobacterium in the enviroimient is associated with irritant contact dermatitis, as well as eye and respiratory irritation. In an effort to understand the potential exposure of humans to the toxins of Z. majuscula the spatial and temporal distribution of LA and DAT were made in Eastern Moreton Bay and Northern Deception Bay. Not all samples of L. majuscula contained LA or DAT. More than ten-fold differences in DAT concentration were found at a single time at North Deception Bay (0.93-11.17 mg/kg freeze dry weight).LA was predominantly found at Eastem Moreton Bay while DAT was found only at North Deception Bay. Highest concentrations of toxin occurred when bloom size and density were also at their maximum (131.9 mg/kg LA and 43.0 mg/kg DAT). In an attempt to predict where L. majuscula biomass and toxins were maximal, and hence greatest chance of human exposure, a variety of physical and biotic parameters were obtained. Bloom intensity occurred when water temperature was maximal. Low precipitation periods were noted to occur before blooms. Total and dissolved reactive phosphorus were present at bloom initiation and peaks of chlorophyll-a in the water column were found at the peaks of Z,. majuscula bloom intensity.In an attempt to understand the extent of human exposure to the toxic effects of L. majuscula several epidemiological studies were completed. To assess the potential affects a survey of the health of ocean users in the North Deception Bay area, a residential area close to L. majuscula blooms, was undertaken. A postal survey was mailed to 5000 residents and a response rate of 27.4% was achieved.High numbers of people (78.2%) responding to the survey reported marine recreational water activity in Moreton Bay. Of those having marine recreational water activity, 34.6% reported at least one symptom, with skin itching the most reported symptom (22.7%) while fever was the least (0.4%). Younger participants had greater water exposure and symptoms than older participants. Participants with greater exposures were more likely to have skin and eye symptoms, suggesting agents in the marine enviroimient contributing to symptoms. Of those entering Moreton Bay waters 29 (2.7%) reported severe skin symptoms, 12 of who attended health professionals. Six (0.6%) reported the classic symptoms of recreational water exposure to L. majuscula, severe skin symptoms in the inguinal region. Participants with knowledge of L. majuscula reported less skin, gastrointestinal and fever and headache symptoms.