solute particles, R$_{1}$/R$_{2}$ = 100 ; agreement is less convincing for R$_{1}$/R$_{2}$ = 10 . This is the value of λ we would predict by matching the drift velocity predicted in a “naive” osmotic view, in which the colloid moves because of an imbalance in the number of solute collisions across its surface, with the colloid velocity, expressed as a function of λ , predicted by hydrodynamic theory. When we simulate a passive colloid in an externally imposed concentration gradient of solute particles, we also find good agreement with the hydrodynamic theory prediction with λ $_{= (}$√$_{2}$$_{/}$$_{3)}$$_{R}$$_{1}$ for R$_{1}$/R$_{2}$ = 100 , but for R$_{1}$/R$_{2}$ = 10 , the diffusiophoretic mobility is smaller than might naively be predicted from osmotic pressure arguments. We speculate that this poor agreement for large solutes may be due to local fluctuations of the solute con- centration field, neglected in the theory. We can also make a quantitative comparison, for small chucking rates, between our MC and Langevin simulation results. For R$_{1}$/R$_{2}$ = 100 , the two simulation schemes produce effective diffusion constants in good agreement, for a choice of Langevin prefactor of A = (4 / 3) πR 3 $_{1}$k$_{B}$ T , corresponding to λ $_{= (}$√$_{2}$$_{/}$$_{3)}$$_{R}$$_{1}$$_{.}$ One of the initial motivation for this study was to investigate possible effects of ex- opolysaccharide production on bacterial diffusivity. The bacterium X. campestris produces xanthan polymer at a rate as high as 10 4 polymer coils per cell per second (as estimated from the data given in Refs. [18, 19]). Assuming a ratio R$_{1}$/R$_{2}$ = 10 − 100 , corresponding to a polymer gyration radius of 10-100 nm, hence to a value of k$_{c}$R 2 $_{2}$/D$_{1}$ ∼ 1 − 100 , and referring to Fig. 5, we find that the effective diffusion constant of the chucker should be significantly different from the infinite dilution value at this chucking rate. On the other hand, the bacterium Sinorhizobium meliloti , a less prolific producer of exopolysaccharide, produces less than 1 polymer per bacterium per second [38], leading to a value of k$_{c}$R 2 $_{2}$/D$_{1}$ of 10 − 4 − 10 − 2 (assuming the same dimensions for the polymer), so that for this bacterium, we expect the effect of polymer production on diffusion to be essentially perturbative. It would be interesting to compare the diffusion constants of mutants of X. campestris which are altered in their exopolysaccharide production rates: our simulations suggest a change in D$_{eff}$ of up to about 50% if exopolysaccharide production were to be eliminated. One would need to be careful in such an experiment to avoid the buildup of polymer in the bacterial suspension, leading to crowding and a subsequent decrease in diffusion. The chucker is an example of a nonequilibrium, “active” system, in which statistical mechanical relations such as the Fluctuation-Dissipation theorem (FDT) may not hold. Comparing the effective diffusion constant D$_{1}$ to a mobility coefficient computed by dragging 17