An one-dimensional differential model of the vertical structure of the turbulent active layer of the sea based on the theory of the rotationally anisotropic turbulence is proposed. The present article consists of two parts. The first part gives the definition of the class of rotationally anisotropic turbulence (RAT) and presents the basics of the theory of RAT. In the second part the theory is used to describe the evolution of the vertical structure of the turbulent active layer of the ocean within the framework of the traditional model assumption about the horizontal homogeneity of all the fields under consideration. A set of submodels (including the analogue to the classical mixing length model) is derived and discussed. The model is used under different initial and boundary conditions. Depending on the actual conditions the model describes various effects: the appearance of the upper mixed layer with a steep thermocline below it, the switching between the convective and diffusive mixing regimes, the staircase-like structure of the thermocline (caused by unsteady shear flow or by double-diffusion instabilities), and the modulation of the velocity field by the period of the oscillation of the heat flux. The set of the model coefficients is determined on the basis of sea surface temperature data from the ocean weather station (OWS) Papa. The results of the following computations of temperature distribution show a reasonable similarity between the model results and the corresponding data from the OWS Papa and the Long-Term Upper Ocean Study (LOTUS). ¿ American Geophysical Union 1996