  Optical seeing measurements carried out at Sierra Negra, the site of the
Large Millimeter Telescope, are reported. The site, one of the highest peaks of
Central Mexico, offers good coverage of Northern and Southern hemispheres and
we have undertaken several campaigns to investigate the astronomical potential
of the site in the optical. Here we report on our campaign aimed at
establishing the seeing quality of the site. We present data of the first three
campaigns of optical seeing monitoring covering from February 2000 to May 2002,
carried out with a Differential Image Motion Monitor. The results clearly
indicate a sub-arcsec seeing, better statistics during the dry season and no
dependence with the time of night. We find no dependence of our results with
the integration time used.
