CHICAGO, Jan. 14 (AP) - F.B.I. agents arrested the reputed Chicago mob boss Joseph Lombardo on Friday in suburban Elmwood Park, nine months after federal prosecutors charged him and 13 others with plotting at least 18 unsolved killings.

The authorities said Mr. Lombardo, whose nickname is the Clown, might never have left the Chicago area, and might have moved among the homes of trusted associates.

While missing, Mr. Lombardo wrote letters with local postmarks to his lawyer and to James B. Zagel, the judge who is presiding in the case.

Mr. Lombardo's lawyer, Rick Halprin, said Saturday that his client was not guilty.

One of Mr. Lombardo's co-defendants, Frank Schweihs, known as the German, was arrested on Dec. 16 after eight months on the lam.