The gross chemical composition and fatty acid composition of 18 species of tropical Australian microalgae (2 cryptomonads, 8 diatoms, 3 prasinophytes, 4 prymnesiophytes and a rhodophyte) grown under standard conditions (pH 8.3"0.2, temperature 25"18C, photon flux density 80"2 m mol my2 sy1 and salinity 25"1‰ and harvested in late log phase of growth., were determined as part of a program to find species for use as feed in tropical aquaculture. Protein was the major component of all speciesrange 24.2–47.1% dry weight.except diatoms range 19.4–36.7% dw.. Highest lipid contents were found in Cryptomonas sp. CRFI01 and Isochrysis sp. NT1422.0 and 23.4% dw, respectively.. The highest percentage of soluble carbohydrate was found in Rhodosorus sp. CS24920.2% dw., while diatoms had the highest amounts of ash range 21.2–38.8% dw.. Moderate levels of the polyunsaturated fatty acidsPUFA. eicosapentaenoic acidEPA.and docosahexaenoic acidDHA.were found in Cryptomonas sp. CRFI0112 and 6.6% of total fatty acids, respectively.. Moderate levels of EPA were also found in Chaetoceros sp. CS256 and in Skeletonema species GOC27 and GOC36. DHA was most abundant in Isochrysis sp. NT149.9% total fatty acids.and Cryptomonas sp. CRFI016.6% total fatty acids.. The highest amount of arachidonic acid AA. was found in Fragilaria sp. GOC52.