The present paper describes the results of a study on the salinity tolerance of two freshwater fishes, Gambusia aftinis (Baird & Girard) and Heteropneustes fossilis (Bloch). The two species were treated with various concentrations of sea water and three most common salts found in the saline waters of Iraq, namely sodium chloride, calcium chloride and potassium chloride. The mortality rates following these treatments were recorded. Both species showed the highest degree of resistance for sea water and sodium chloride, and the lowest degree of tolerance to potassium chloride. Gainbrtsia afinis was more resistant to these salts and sea water than IT. fossilis.

The testes of Barbus luteus are a pair of elongated structures composed of an anastomosing system of seminiferous lobules. Six spermatogenetic stages of germ cells have been described in the gonad. The endocrine factor lies in the interstitial cells. Spermiation occurs during the months of May, June and July. Fertilization appears to be external. Un‐expelled spermatozoa undergo pycnotic degeneration. The testes show a regular annual cycle which has been described in five arbitrary stages.

Age, growth, length‐weight relationship, sex ratio, stages of maturity and fecundity of the greenback mullet in Shatt Al‐Basrah Canal, an estuary in Southern Iraq, were studied from February 1985 to January 1986. Age and growth determinations were made from scales from 538 fish ranging from 145 to 310 mm total length. Females (age groups I to VI) were slightly longer than males (age groups I to IV) and their growth, analysed by length‐weight relationship, followed very closely the cubic‐law for isometric growth, but males grew more slowly. The annual average male: female sex ratio was 1.0: 1.4. Monthly examinations of stages of maturity produced abundant gravid fish in February. The maximum values for the gonadosomatic index were obtained in February and March. The smallest mature male and female were respectively 137 and 142mm long. The fecundity ranged between 133 224–295 065 for fish ranging from 182 to 243 mm total length. Although eggs or newly hatched larvae were not found in the Canal, fry 27–40 mm in length were captured from April to June. Spawning appears to occur in the Arabian Gulf, not far from Shatt Al‐Basrah Canal.

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).

The present paper describes the morphology, histology, and annual cyclical changes in the ovaries of Burbus luteus (Heckel). The ovaries are paired structures, joined by short oviducts. The ovarian structure consists of a number of lamellae, which contain the germ cells in various phases of maturation-immature, maturing and mature. Two types of yolk have been reported in the oocyte. The endocrine function of the gonads has been attributed to the follicular cells forming the ' pre-ovulatory corpus luteum ' and ' postovulatory corpus luteum '. The ovaries undergo a regular annual cycle which has been divided into five stages, based on various criteria, vlz, ovarian size, colour, gonosomatic index, average ova-diameter, histological composition, and the spawning activity.