Previous double team experiments testing line transect methodology for harbor porpoise indicated that a relatively small proportion of the sightings were actually seen by both teams. Results of an additional field experiment designed to test the effect of concentrating additional observer effort along the transect line are presented. Observer effort was varied by rotating two extra observers between two independently searching teams of three members each and instructing the two additional observers to concentrate their searching effort on the trackline. This additional effort had a differential effect on the two teams. For one team, the distribution of perpendicular sighting distances became highly concentrated on the trackline when the additional observers were present while for the other team the additional observers had little effect. Difficulties were encountered in determining which sightings were seen by both teams, particularly on transects in which the density of sightings was high. However, the actual proportion of duplicates, at least for the lower density transects, appears to be low and the additional observers did not have a consistent effect on the duplication rate between the two teams. A large amount of heterogeneity also existed among the individual observers based on comparisons of the distribution of perpendicular sighting distances when each individual observer occupied the center viewing position and based on the actual number of sightings detected by each observer.