Shares of

PSINet

(PSIX)

plummeted by more than 50% Thursday after the company posted narrower-than-expected third-quarter losses but warned that fourth-quarter results would not meet expectations.

The the e-commerce infrastructure and service provider also announced the resignation of President and Chief Operating Officer Harold Wills.

Excluding items and charges, PSINet posted an operating loss of $182 million, or 95 cents a share. Fifteen analysts polled by

First Call/Thomson Financial

expected a loss of $1.28 a share. Including extraordinary charges and items, the company lost $1.4 billion, or $7.34 a share, compared with a loss of $88.7 million, or 68 cents a share, in the year-ago period.

Third-quarter revenue was $352.5 million, up 151% from $140.6 million a year earlier.

PSINet, based in Ashburn, Va., said its board of directors is in talks with several candidates to fill the vacancies left by Wills' departure. The company also said its 80% stake in

Xpedior

(XPDR)

was classified as a discontinued operation in third-quarter results.

Share of PSINet were recently at $3.12 in

Nasdaq

trading, off 54% from the previous day's close.