During 6 cruises along the 1 lO'E from 32°S to 9*S in the Indian Ocean, at about 22.30 hr, from surface to 200 m, 88 oblique hauls have been done, with a 5 foot Isaacs-Kidd midwater

trawl having a nylon mesh no. 2 (aperture 0,366 mm) in the cod-end.

One hundred species of calanoid Copepods are listed. The 39 most important are numbered. With the object of doing a comprehensive study of the ecosystems in position and there time evolution during one year, the specific diversity is evaluated at the 88 stations. The diversity is measured with the information theory formulae (Shannon, 1948)

I = _n  Pi logjpi (n is the number of species and pi the proportion of the total number of individuals which belong to the i th species). We use also, two other values derived from this diversity indices and measuring the ' equitability' (Lloyd and Ghelardi, 1964).

Between 32°S and 9°S the usual increasing gradient of species diversity from the poles towards the equator, is masked by some hydrological cycles.

Related to the hydrological conditions, we note that the less diversified communities are not only in the south but also between 15°S and 24°S associated with the tropical water where this one overlaps the subtropical water.

All year long, at the boundary of the subtropical oxygen minimum water and the tropical oxygen minimum water, near 25''S, it appears that mixed communities with high diversity (owing to the meeting of the southern region) spread out.

Biologically the diversity accounts well for seasonal variations in the south of the transect. The high values of the 'equitability' (more than 70% on an average) from May to July shows that between 20''S and 30°S the Copepods community reaches its most mature level. This last stage of succession for our taxocenose coincides with the initial stage of phytoplankton population characterised by an important amount of pigments (Chi a and Chi c) and a very high productivity rate. Later appear the periods of the maximal abundance of Copepods followed by Phronima (Repelin 1970) Euphausiids (Roger 1966) or macroplankton and micronekton (Legand 1969). In the northern part the sequences of this succession are seasonally less well marked, the different stages are shorter and the maximum abundance lasts longer.