A new equation, which enables the prediction of the mean value of the friction factor during the flow of a thixotropic fluid in a horizontal rectilinear cylindrical pipe, was obtained using a method proposed by Kemblowski & Petera. According to the obtained equation, the pressure drop is a function of three dimensionless numbers: the generalized Reynolds number Re standing for the ratio of inertial to viscous forces for an Ostwaldian fluid, a modified memory effects number De and a ＇structural＇ number Se which is correlated to the maximum breakdown of structure of the fluid. This relation is only valid for laminar flows and is based on several hypotheses described in the present article. The proposed rheological model is based on a structural approach, featuring a rheological state equation describing shear stress, and a structural decay equation. The fluid is stirred yogurt and its structural parameter lambda follows a second order kinetic equation. Experimental validation of the friction factor formula showed good agreement.