The genome of the salt-lake archaeon Haloferax volcanii shows some unusual characteristics, including polyploidy and three megaplasmids. We identified a strain spontaneously cured of the megaplasmid, pHV1. pHV1 is thought to have been acquired by a recent horizontal gene transfer [1], and we found that it was not required for apparently normal growth in standard high-salt media. However, the pHV1-cured strain showed increased cell motility. Furthermore, upon dilution of cultures growing in low salt stress conditions, pHv1 conferred a substantial growth lag and a greatly reduced ability to form colonies. These cultures eventually attained a normal mid-log growth rate, suggesting that culture dilution in low-salt conditions triggers pHV1-dependent quorum sensing mechanisms that inhibit growth. Expression of certain regions of pHV1 is dependent on the salinity of the media, [2,3], and we are investigating how these may control the pHV1-dependent phenotypes.