The study presents an experimental analysis of pressure drop of a fixed bed isomerization bioreactor where washing out and ageing of biocatalyst particles take place under conditions approximating industrial practice. The principal relationship DELTAp vs. U(L) is investigated at various initial bed compressions between 20 and 60 kPa and at various size distribution with mean particle size between 0.4 and 0.7 mm. The effect of biocatalyst deterioration upon the dynamic resistance is evaluated by model systems and found to yield a 30 % increase of DELTAP compared to the fresh biocatalyst performance at liquid velocities exceeding 0.3 cm/s. Higher bed compaction is registered in beds of monodisperse immobilized biocatalyst preparation or polydisperse preparations with similar amount of particles of different size. Higher voidage and lower DELTAp is registered in the intermediate case where the particle size distribution is largely non-uniform. A shift of DELTAp is observed and interpreted as dynamic bed restructuring and a micro-step of bed clogging following the increase of fluid compression.