It's all in the family, unfortunately.

Shares in

Hotels.com

(ROOM)

tanked Monday after the online reservation service cut estimates, blaming lower-than-expected room rates.

Other online travel stocks took it on the chin, and Hotels.com executives spoke of broad weakness in the lodging market. But the brunt of the collateral decline took place at Hotels.com's parent, the

USA Interactive

(USAI) - Get Report

group run by media mogul Barry Diller, and related companies.

Thus, investors may believe that the sudden room-price deflation of which Hotels.com spoke Monday is already reflected in the stocks of other lodging and related companies, both online and off. Or they may be getting worried that the decline at Hotels.com -- which said fourth-quarter and 2003 brand advertising and payroll expenses would be bigger than previously envisioned -- may signal that other USAI companies will have trouble keeping their costs under control.

By early Monday afternoon, Hotels.com's shares had fallen $15.38 to $43.66, down 26%. USAI, which said Monday that full-year 2002 performance will be "generally in line" with "key bottom-line metrics" despite Hotels.com's preannouncement, saw its shares fall $3.01, or 12%, to $21.20. USAI's other publicly traded online travel reservations subsidiary,

Expedia

(EXPE) - Get Report

, fell $7.69, or 11%, to $63.21. USAI's

Ticketmaster

(TMCS)

, which isn't even an online travel company, fell $2.92, or 13%, to $19.70.

Meanwhile, other online travel companies didn't suffer quite as badly as Hotels.com's relatives.

Priceline.com

(PCLN)

, already down 75% over the last year, fell 4 cents to trade at $1.56. The U.K.-based

ebookers

(EBKR)

fell 71 cents, or 2.5%, to $11.69.

The primary cause of the fourth quarter's bad news, according to Hotels.com executives during a Monday morning conference call, was an average daily rate for hotel bookings that fell suddenly in mid-November and stayed down, contrary to expectations.

In previous years, those average daily rates for the fourth quarter have risen significantly from third-quarter levels. But this year, said CEO David Litman, daily rates fell from the low to mid-$120s in October and early November to the low $110s. "And it happened fairly quickly," he said.

Rather than seeing the $10 daily increase the company had forecast for 2003, Hotels.com now believes rates will stay flat, cutting $100 million from the company's expected revenue for 2003. Separate from that figure, the company is cutting its revenue budget by $50 million, "based upon some current softness in the travel market that reflects the near-term possibility of war in Iraq, particularly in international destinations." The company is now forecasting $1.25 billion in 2003 revenue, down from the $1.4 billion it forecast only two months ago.

Based on expectations of growth in revenue and transactions in 2003, the company says it's increasing its technology budget. It will also be spending more money than previously budgeted for brand advertising, connected in part with a push into Europe.

Meanwhile, major hotel operators seemed unhurt by Hotels.com's discussion of rumors of war.

Starwood Hotels

(HOT)

fell 68 cents, or 3%, to $24.05.

Marriott International

(MAR) - Get Report

fell 36 cents, or 1%, to $33.92.