A study has been made of the distribution of polycycloaromatic hydrocarbons (PAH) in bottom sediments of the southern basin of Lake Baikal, It has been shown that perylenes are confined to diatomaceous sediments of the lake shelf and have a diagenetic origin from biogenic sources; the origin of other PAH with pericondensed structures not containing substituents is assumed to be connected with hydrothermal oil shows.