  Stellar rotation periods are valuable both for constraining models of angular
momentum loss and for under- standing how magnetic features impact inferences
of exoplanet parameters. Building on our previous work in the northern
hemisphere, we have used long-term, ground-based photometric monitoring from
the MEarth Observatory to measure 234 rotation periods for nearby, southern
hemisphere M dwarfs. Notable examples include the exoplanet hosts GJ 1132, LHS
1140, and Proxima Centauri. We find excellent agreement between our data and K2
photometry for the overlapping subset. Amongst the sample of stars with the
highest quality datasets, we recover periods in 66%; as the length of the
dataset increases, our recovery rate approaches 100%. The longest rotation
periods we detect are around 140 days, which we suggest represent the periods
that are reached when M dwarfs are as old as the local thick disk (about 9
Gyr).
