The components of 4 major groups of microplankton were identified and their numerical abundance determined in net samples collected at depth intervals down to 600 m at a permanent station off the H. Steinitz Marine Biological Laboratory, Elat. The samples analyzed were collected once a fortnight over a period of 1 year beginning in June 1974. The groups studied were the Cyanophyta, Bacillariophyta and Pyrrhophyta of the phytoplankton and the Tintinnina of the microzooplankton. The pattern of vertical distribution of the phytoplankton as a whole showed a general decrease in cell numbers with increasing depth. The blue-green algae, consisting mainly of Trichodesmium sp. trichomes, were confined primarily to the upper 100 m. The diatoms were unevenly distributed, with one species, a minute centric diatom, Thalassiosira subtilis, associated with a massive bloom during March 1975 between 300 and 400 m. The peridinians, the group with the largest number of species, included forms which were evenly distributed throughout the whole water column and forms limited either to the upper or deeper water strata in accordance with their light intensity preferences. The mass occurrence of newly-emerged dinoflagellate cysts of Pyrophacus horologicum, a weakly-armoured dinoflagellate, in the 200 to 300 m depth interval during April 1975, suggests that reproductive processes in dinoflagellates may also be light-controlled. The tintinnids, like the phytoplankton groups, were most abundant in the upper 100 m with a gradual decrease in numbers of individuals, though not in species, in the deeper water strata. The overall yearly pattern of microplankton distribution indicates 3 peaks: late fall and early summer peaks consisting primarily of blue-green algae and one in early spring consisting of several species of diatoms and peridinians and of species of tintinnids which thrive in the same niches as the phytoplankton. Both phytoplankton and tintinnid production was lowest during the summer months.