A new matrix factorization program 'CHEMTAX was used to ~nterpret high-performance hquid chromatography (HPLC) pigment data from a transect between Prydz Bay Antarct~ca and Australla dunng March 1987 The program calculated the abundance of diatoms dinoflagellates haptophytes resembling Emiliania huxleyl haptophytes resembling P h a e o c j~s t~s antarctlca cyanobacteria prasinophytes, chlorophytes and cryptophytes along the transect The results were compared with those of microscopy and particle size analysis The transect was dominated by small cells particle size analysis showed that particles < 2 pm represented 27 to 44 Oh of the total by number while particles 2 to 20 pm represented 55 to 68% Particles >20 pm never represented more than 3 % by number but constituted 57 to 93 % of the total v o l u n~e Microscop~c analysis showed that small flagellates were the most abundant cells along the transect wlth a 5-fold increase In abundance at 47"s Numbers of dlatoms (most <20 pm in size) Increased markedly south of the Polar Front, correlating with the concentration of silica D~noflagellate numbers were relatively constant along the transect although somewhat higher north of 5 0 " s Those <20 pm in size were most numerous and accounted for most of the latltudinal vanation Interpretation of HPLC pigment data using the CHEM TAX program was consistent with microscopical analysis The computed abundances of diatoms and dinoflagellates correlated more strongly with the numbers of small (<20 pm) dlatoms and d~noflagellates, respectively than with large ones Computed cyanobactenal abundances correlated well wlth microscopical observations except for small errors where cyanobactena were absent probably d u e to m~sallocation of zeaxanthln from chlorophvtes and praslnophytes The program was able to distinguish 2 populations of haptophytes along the transect representing Phdeocystis antarctlca and coccol~thophonds even though their pigment compositions were qualitatively (though not quantitatively) Identical It also indicated the separate distributions of chlorophytes and prasinoxarithin-containing praslnophytes, and showed the presence of cryptophytes where none were observed by microscopy