Weak localization was observed and determined in a black phosphorus (bP) field-effect transistor 65 nm thick. The weak localization behaviour was found to be in excellent agreement with the Hikami-Larkin-Nagaoka model for fields up to 1 T, from which characteristic scattering lengths could be inferred. The dephasing length L_ϕ was found to increase linearly with increasing hole density attaining a maximum value of 55 nm at a hole density of approximately 10^13 cm^-2 inferred from the Hall

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... . The temperature dependence of L_ϕ was also investigated and above 1 K, it was found to decrease weaker than the L_ϕ∝ T^-1/2 dependence characteristic of electron-electron scattering in the presence of elastic scattering in two dimensions. Rather, the observed power law was found to be close to that observed previously in other quasi-one-dimensional systems such as metallic nanowires and carbon nanotubes. We attribute our result to the crystal structure of bP which host a 'puckered' honeycomb lattice forming a strongly anisotropic medium