<p>We study a recently discovered carrollian bosonic string, described classically by a sigma model where both the worldsheet and the target spacetime are carrollian. After fixing the carrollian analogue of the conformal gauge, we determine the Lie algebra of residual gauge symmetries and show it is isomorphic to the three-dimensional extended BMS algebra. We quantise the sigma model à la BRST and determine the spectrum of the corresponding string theory by computing the BRST cohomology. In contrast with the usual bosonic string, the spectrum of the carrollian string is finite-dimensional. The cohomology displays Poincaré duality and can be interpreted, for a given momentum, as inducing representations for unitary irreducible representations of the 26-dimensional Carroll group. Furthermore we interpret (most of) the cohomology as deformations of the spacetime carrollian structure augmented by the Kalb-Ramond field to which the carrollian string couples.</p>