The copper resistance of a strain of Escherichia coli isolated from the effluent of a piggery where pigs were fed a diet supplemented with copper sulfate was controlled by a conjugative 78-megadalton plasmid designated pRJ1004. Plasmid pRJ1004 exhibited surface exclusion and incompatibility with standard plasmids belonging to incompatibility groups I1 and K. Sensitive strains of E. coli K-12 were unable to form colonies on nutrient agar containing more than 4 mM copper, whereas transconjugants which harbored pRJ1004 were able to form colonies on medium containing up to 20 mM copper.