  The dynamics of fluids deep in stellar interiors is a subject that bears many
similarities with geophysical fluid dynamics, with one crucial difference: the
Prandtl number. The ratio of the kinematic viscosity to the thermal diffusivity
is usually of order unity or more on Earth, but is always much smaller than one
in stars. As a result, viscosity remains negligible on scales that are
thermally diffusive, which opens the door to a whole new region of parameter
space, namely the turbulent low P\'eclet number regime (where the P\'eclet
number is the product of the Prandtl number and the Reynolds number). In this
review, I focus on three instabilities that are well known in geophysical fluid
dynamics, and have an important role to play in stellar evolution, namely
convection, stratified shear instabilities, and double-diffusive convection. I
present what is known of their behavior at low Prandtl number, highlighting the
differences with their moderate and high Prandtl number counterparts.
