  M dwarfs are low-mass main-sequence stars, the most numerous type of stars in
the solar neighbourhood, which are known to have significant magnetic activity.
The aim of this work is to explore the dynamo solutions and magnetic fields of
fully convective M dwarfs with varying magnetic Prandtl numbers ${\rm Pr_M}$,
and a rotation period (${\rm P_{rot}}$) of 43 days. ${\rm Pr_M}$ is known to
play an important role in the dynamo action; dynamos for low-$\rm Pr_M$ and
large-$\rm Pr_M$ have very different properties. We performed three-dimensional
magnetohydrodynamical (MHD) numerical simulations with the ``star-in-a-box''
model using stellar parameters for an M5 dwarf with $0.21 M_{\odot}$. We found
that the dynamo solutions are sensitive to ${\rm Pr_M}$. The simulations at
this rotation period present periodic cycles of the large-scale magnetic field
up to ${\rm Pr_M} \leq 2$; for higher values the cycles disappear and irregular
solutions to arise. Our results are consistent with previous studies and
suggest that the dynamos operating in fully convective stars behave similarly
as those in partially convective stars.
