HOUSTON ( TheStreet) -- Shares in Continental ( CAL) led the airline industry down Friday after the carrier said its unit revenue in June was lower than analysts had estimated.

Continental's revenue per available seat mile (RASM) is one of the industry's most closely watched indicators because historically, Continental has been one of the few carriers to report RASM as part of its monthly traffic report. It remains the first to report RASM for June.

The increase in Continental RASM came in between 21% and 22% the airline said, while analysts had estimated 23% to 26%. Shortly before the close, stock in both Continental and proposed merger partner United ( UAUA) was down about 9%. Continental was trading at $20.27, down $1.89. United was trading at $18.82, down $1.90. Meanwhile, shares in Delta ( DAL) were down 64 cents to $11.09.

In a report Friday, CRT Capital Group Mike Derchin wrote, "while a very strong result, June was lower than our forecast of 23-24% growth and slightly below Continental's 23.4% May PRASM increase." Derchin estimated that Continental's yield, or revenue per revenue passenger mile, rose by 19%, below the 20% he had expected.

Meanwhile, JP Morgan analyst Jamie Baker said he had estimated 25% for Continental RASM. When the number came in low, he reduced his second quarter earnings estimate for the carrier to $1.52 from $1.70. Analysts surveyed by Thomson Reuters are estimating $1.51.

Baker cautioned that "the knee-jerk reaction from some may be to interpret CAL's 'miss' as evidence of slackening demand," a thesis that he does not accept. Recent quarterly guidance from the big three carriers "broadly topped expectations at the time," he said. Moreover, high estimates may have reflected "overzealous sell-side expectations," including, Baker said, his own.

In the coming week, Baker expects reports of RASM growth of 30% at United, 25% at Southwest ( LUV) and 20% at US Airways ( LCC).

-- Written by Ted Reed in Charlotte, N.C.