Comcast had enjoyed rapid subscriber growth in the last 18 months due to its triple-play bundles of video, high-speed Internet and phone services but executives said competition had increased significantly from satellite and telephone operators.
"We're seeing increasing competition and a softer economy and as a result a slightly slower growth rate," Chief Executive Brian Roberts said on a conference call.
Chief Operating Officer Stephen Burke said competitors are winning customers by offering aggressive price discounts.
Comcast shares were down $1.86, or 7.8 percent, to $21.99 on the Nasdaq Thursday morning after hitting a year low of $21.80. The company said it lost more than 65,000 basic video subscribers in the third quarter, exceeding the loss of 25,000 forecast by Goldman Sachs.
Analyst Thomas Eagan of Oppenheimer said Comcast had missed most of his forecasts on key third-quarter metrics due to several factors.
"They lost more basic subscribers and added less digital subscribers than we expected," said Eagan. "It's a combination of problems in their acquired cable systems, some competition and a weaker housing market."
Third-quarter net profit fell to $560 million, or 18 cents a share, from a year ago's $1.217 billion, or 38 cents per share, which included one-time transaction gains. Excluding gains Comcast's income rose 2 percent. Profit was in line with analyst expectations.
The company said revenue grew 21 percent to $7.781 billion, as the basic video subscriber losses were offset by growth in phone, Internet and digital video customers.