Security software maker

Symantec

(SYMC) - Get Report

plans to acquire privately held data loss-prevention company

Vontu

for $350 million in cash and assumed options.

The acquisition, expected to close in the fourth quarter, expands Symantec's portfolio into one of information technology's strongest growth segments.

Symantec's acquisition comes months after similar purchases by rivals

Websense

(WBSN)

and

McAfee

(MFE)

.

Last year, McAfee bought startup

Onigma

, while Websense acquired

Port Authority Technologies

for $90 million.

Recently, RSA, a division of

EMC

(EMC)

, acquired data loss prevention specialist

Tablus

for an undisclosed amount.

Research firm IDC estimates the information protection and control market will grow at 33% compounded annual growth rate to reach $3.2 billion by 2011.

Shares of Symantec closed Monday off 15 cents to $18.36.