The Almeria-Oran front forms where surface waters of Atlantic and Mediterranean origin meet at the eastern end of the Alboran Sea. A multidisciplinary field experiment on RRS Discovery in December 1996, in the second observational phase of the EU-funded Observations and Modelling of Eddy scale Geostrophic and Ageostrophic motions (OMEGA) project, observed the biological impact of mesoscale frontal instability of the Almeria-Oran frontal jet. It is concluded that periodic vertical velocities of ∼ 20 m/day, associated with the propagation of wave-like meanders along the front, have a significant effect on the vertical distribution of zooplankton across the front despite their ability to migrate at greater speeds. Observations of a layer of fluorescence coincident with subducted surface waters indicated that phytoplankton were drawn down and along isopycnals, by cross-front ageostrophic motion, to depths of 200 m. From the study of sound-scattering layers (SSL) identified in acoustic backscatter data, a layer of zooplankton was found coincident with the drawn-down phytoplankton. This layer persisted during and despite diel vertical migration. High-resolution optical plankton counter (OPC) data showed smaller zooplankton, which did not undertake diel vertical migration, remained concentrated near the surface in the fast-flowing frontal jet. © 2001 Elsevier Science B.V. All rights reserved.