The study was conducted on the Shatt Al-Arab River at Basrah, Iraq from September 1976 to August 1977 at three stations located at the upstream, middle and lowest parts of Basrah city. There was a bimodal seasonal variation of chlorophyll-a, the concentration of which ranged between0.52-3.25 mg/m3. Thegross primary production ranged between 6.03-37.02 mgc/m3/hr and showed a unimodal seasonal variation with a maximum in August. From the concentration of chlorophyll-a and from measurement of primary productivity it was clear that the section of the river at the upstream end of Basrah city was poorest and that at the middleof the Basrah city below Ashar Channel was the richest. A positive corelation between primary productivity and chlorophyll-a.

In the Ashar Canal in the city of Basrah and in the Shatt al‐Arab directly below the mouth of the Ashar Canal algological and saprobiological investigations were carried out. The water of the Ashar Canal is β‐ to α‐mesosaprobic, the water of the Shatt al‐Arab below the mouth of the Ashar Canal is β‐mesosaprobic.

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SUMMARY. The ecology of three commonly occurring cyprinodonts, Aphanius dispar (Ruppell), A. sophiae (Heckel) and A. mento (Heckel) was studied in the Lower Mesopotamian Plain of Iraq. These fishes are chiefly herbivorous, live in the same habitat and generally take the same food, although the dentition of the jaw and pharynx in A. dispar differs from that of A. mento and A. sophiae. The gut contents were mostly filamentous algae. In laboratory experiments all three species ate Gambusia embryos. A. dispar and A. sophiae ate mosquito larvae whereas A. mento did not. All three species were infected with a cestode parasite which prevented development of female ovaries. The behaviour of the three species was different. Both A. dispar and A. sophiae form schools, swimming and feeding together. In their natural habitat, males and females of A. dispar were in separate schools. A. mento did not form schools; the males of this species displayed aggressive behaviour. Some observations were also made on Gambusia affinis (Baird & Girard).

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