Natick-based Middlesex Savings Bank has officially completed its acquisition of and merger with Medway’s Strata Bank. 

At the same time, Middlesex Bancorp MHC, the parent mutual holding company of Middlesex Savings Bank, completed its acquisition of Service Bancorp MHC and Service Bancorp Inc., the parent holding companies of Strata.

The transactions, initially announced in December 2008, create a $4 billion financial institution serving more than 100,000 customers throughout eastern Massachusetts. The combined bank will have 31 branches, 55 ATMs and four commercial banking offices in 30 towns throughout the region. 

Following the merger, Middlesex Savings Bank will be the eighth-largest bank in Massachusetts, and the second-largest mutual bank in the state.

"This merger is a natural combination for both institutions, since we share a common community heritage and commitment to customer service," said John R. Heerwagen, president, chairman and CEO of Middlesex Savings Bank. "Strata customers will enjoy much greater convenience through an expanded network of branches and ATMs.  In addition, they will have access to a broader range of products and services as well as access to increased lending capacity."

Strata Bank and Middlesex Savings Bank offices will continue to operate separately until the conversion weekend of Aug. 15 and 16.  On Aug. 17, all Strata locations will open as Middlesex Savings Bank, and former Strata customers will be converted to Middlesex accounts and systems. Strata customers will be receiving conversion information in mid-July.