There is no generally applicable technique for finding the position of an autonomous mobile robot operating in an un structured or varying environment. This article describes a project to investigate the use of short-lived navigational mark ers consisting of olfactory chemicals to guide mobile robots and to help them search and explore efficiently. Such a method of laying a trail to mark the path of a robot provides differ ential navigational information relative to the starting point and starting direction. Many insects employ olfactory cues as navigation aids and to improve their efficiency when searching for food. Three scenarios are proposed in which navigational markers can be of assistance to a mobile robot. The design and characteristics of an adsorbed mass olfactory sensor are described. This sensor was designed to be mounted on a mo bile robot and is suitable for detecting and tracking olfactory chemicals. The performance of this sensor is described together with preliminary results of using it to track an odor trail.