Pseudomonas pickettii strain US321 appeared to elicit a copper resistance mechanism upon exposure to copper. The bacterial colonies turned blue in the presence of this metal in a chemically defined medium, suggesting accumulation of copper. Prolonged exposure to copper resulted in the characteristic ''copper sink'' morphology of the colonies. Atomic absorption spectrophotometric analysis confirmed that this organism can accumulate copper. The strain US321 exhibited a high-molecular-weight plasmid, pUS321, The plasmid-cured strain, PC25, is highly sensitive to copper owing to a poor copper management, A plasmid-encoded sequestration mechanism operating in the strain US321 is suggested.