Parietochloris incisa is an arachidonic acid-rich snow green alga. The main phy siological profiles, such as ash free dry weight (AFDW), chlorophyll, carotenoid , protein and total fatty acids (TFA), in this alga exposed to old culture super natant (OCS) at the decline phase or its crude ethyl acetate extracts (CEAE) wer e investigated by using tubular photobioreactors of different diameters. Results showed that both OCS and CEAE had strong inhibitory effect on the above physiol ogical parameters. The longer the culture was exposed to OCS and the more CEAE w ere added into the algal culture, the more the above physiological properties we re inhibited. Arachidonic acid (AA), the dominant component of fatty acids in th is alga, was also seriously inhibited with respect to total TFA, AFDW of cell ma ss, or culture volume, due to a probable reduction of enzymes activities catalyz ing chain elongation from C18:1ω9 to AA. These results incontestably evidenced t hat some CEAE dissolving substances existing in OCS, like auto-inhibitors, inhi bited P. incisa growth through feedback. Hence, any efficient removal of aut o-i nhibitors from algal culture to decrease their bioactivity could be good for max imal production of desired products like AA.