Barclays' chief executive, Martin Taylor, described Wells Fargo Nikko as "a world leader in its field, an advanced, technology-driven business with strong managment. It is an excellent product and geographical fit."
The company - a joint venture between Wells Fargo, the American bank, and Nikko, the Japanese securities house - made profits last year of $45m and had net assets of $80m. The deal is expected to be tied up by the end of the year, depending on regulatory approval.
WFNIA, a pioneer of index tracking investment, is one of the largest US pension fund managers. In March it had $171bn under management compared with BZW's $83bn.
The combined group will easily outstrip Britain's largest pension-fund manager, Mercury Asset Management, and approaches Fidelity, the world's largest fund manager, which has over $300bn invested.