The accumulation of high levels of the ketocarotenoid astaxanthin by the green freshwater microalga Haematococcus pluvialis in response to extreme environmental conditions is a well recorded phenomenon. However, there is still considerable debate regarding the nature and possible interactions of the abiotic factors that are thought to be primarily responsible for stimulating the synthesis of this carotenoid. In this study H. pluvialis was exposed to a number of different nutritional and physical parameters to determine the effect each exerted on stimulating astaxanthin formation in the alga. When H. pluvialis was cultivated in media deficient in nitrogen, algal growth was limited severely and astaxanthin synthesis greatly stimulated. Similar stimulatory effects on carotenoid synthesis were observed in the presence of elevated levels of ferrous iron and, especially, when the alga was transferred into saline media. In both cases algal growth was once again severely limited. However, the single most important factor in terms of carotenogenesis was to subject the alga to high photon-flux densities. In contrast to these effects, transfer of the alga to phosphate-limiting conditions increased the rate of astaxanthin synthesis but, importantly, algal growth was not inhibited greatly. The extent to which each parameter tested was able to stimulate the formation of astaxanthin in the alga varied and the overall effectiveness of each treatment in promoting astaxanthin formation in the alga is discussed.