An exact similarity solution is obtained for the rise of a buoyant thermal in Stokes flow, in which both the rise height and the diffusive growth scale like t1/2 as time t increases. The dimensionless problem depends on a single parameter Ra = B/(??) – a Rayleigh number – based on the (conserved) total buoyancy B of the thermal, and the kinematic viscosity ? and thermal diffusivity ? of the fluid. Numerical solutions are found for a range of Ra. For small Ra there are only slight deformations to a spherically symmetric Gaussian temperature distribution. For large Ra, the temperature distribution becomes elongated vertically, with a long wake containing most of the buoyancy left behind the head. Passive tracers, however, are advected into a toroidal structure in the head. A simple asymptotic model for the large-Ra behaviour is obtained using slender-body theory. The width of the thermal is found to increase like (?t)1/2, while the wake length and rise height both increase like (RalnRa)1/2(?t)1/2, consistent with the numerical results. Previous experiments suggest that there is a significant transient regime.