Presence of synthetic dyes in water bodies, even at very low concentrations can be highly toxic to living organisms, reducing the growth of different microorganisms and preventing the photosynthesis in aqueous flora. Pseudomonas pseudoalcaligenes and biofilm supported on activated carbon were used for adsorption of acid blue 25 (AB 25). The effect of initial dye concentration (0-300 mg/L), contact time (0- 240 min), and pH values (3.0- 8.0) were studied. The optimum pH value for biosorption of AB 25 was pH 3.0 for the two biosorbents, while contact time was180 min for P. pseudoalcaligenes cells and 210 min for the biofilm. The biosorption isotherm study revealed that both Langmuir and Freundlich models were suitable for biosorption process. The maximum adsorption capacities for AB 25 by P. pseudoalcaligenes and biofilm were 98 and 113.4 (mg/g), respectively. On the other hand, the pseudo-second order was a good model for description the biosorption kinetics. Fourier Transform Infrared FT-IR spectrum analysis showed that the presence of amines plays an important role in biosorption process. According to experimental data, the presence of P. pseudoalcaligenes could be enhanced the capacity of activated carbon to adsorb the dye from aqueous solutions.