correlation functions by maximum likelihood techniques [e.g. @gd89; @jk95; @zaroubi97; @jusf00; @brid01; @abate09]. However, the bulk flow is particularly appealing because of its simplicity and the fact that it is entirely linear for sufficiently large spheres. The constraints from peculiar velocities, including the bulk flow, are unique since they are local at redshifts very close to zero and they directly probe the growth index $\gamma =\frac{\dd \ln f}{\dd \Omega_m}$ where $f$ is the linear growth factor [@Peeb80; @Lind05]. Adopting the WMAP7 cosmological parameters [@wmap7], we derive a local constraint $\gamma=0.495\pm 0.096$. This constraint is completely independent of the biasing relation between galaxies and mass. Further, it is essentially a constraint at $z=0$. In contrast, the lowest redshift constraint obtained from a study of redshift distortions in the 2dF galaxy redshift survey is at $z\approx 0.15$ [@Hawk03]. Our constraint significantly improves on previous constraints on $\gamma$ [@Dos10; @Wei] derived at higher redshifts. This result could help us distinguish between alternative theories for structure formation [e.g. @amend05; @Guz08; @knp10]. For $\sigma_8=0.8$, the constraint on $\gamma$ disfavors DGP models at $\sim 2\sigma $ level, but it is consistent with $f(R)$ gravity models [e.g. @Starob07; @Gann09; @Wu09; @Fu10]. But $\sigma_8$ in these models should 