July 1 (Reuters) - International Business Machine Corp has agreed to buy privately held security software company BigFix Inc, continuing an acquisition spree to expand its line of corporate security products.

The companies did not disclose the terms of the deal, announced on Thursday. It is IBM's 11th security-related acquisition since 2006. The acquisition is expected to close in the third quarter.

BigFix sells software that a company can use to automatically deliver security patches and software updates to hundreds of thousands of personal computers across its network. It constantly monitors the network to find straggler machines that may not have the most recent updates.

BigFix's rivals include Microsoft Corp and Symantec Corp.

IBM shares fell 1.1 percent in midday trading to $122.11, in line with a decline in the Standard & Poor's 500 Index. (Reporting by Jim Finkle; editing by John Wallace)