The combined effects of Navier slip and magnetic field on boundary layer flow with heat and mass transfer of a water-based nanofluid containing gyrotactic microorganisms over a vertical plate are investigated. Using Oberbeck-Boussinesq approximation and similarity transformation, the nonlinear model equations are obtained and tackled numerically to obtain the dimensionless velocity, temperature, nanoparticle concentration and density of motile microorganisms together with the reduced Nusselt, Sherwood and motile microorganism numbers. The present numerical results are compared with available data and are found in an excellent agreement. Pertinent results are presented graphically and discussed quantitatively with respect to variation in the controlling parameters. It is observed that the magnetic field suppresses the dimensionless velocity and increases the dimensionless temperature inside the boundary layers. The bioconvection parameters tend to reduce the concentration of the rescaled density of motile microorganisms. It is also found that the reduced Nusselt, Sherwood and density numbers of microorganisms depend strongly upon the magnetic, buoyancy, nanofluid and bioconvection parameters. (C) 2014 Elsevier Ltd. All rights reserved.