By Max Colchester 

LONDON-- Barclays PLC on Wednesday was fined $115 million by a U.S. regulator for allegedly attempting to manipulate a major benchmark interest rate.

The Commodity Futures Trading Commission said in a statement that Barclays's U.S. traders attempted to manipulate the U.S. dollar iteration of ISDAfix, or the International Swaps and Derivatives Association Fix, between 2007 and 2012. The ISDAfix benchmark is widely used in setting payout rates on pension funds and determining the cost of real-estate loans.

A group of other financial institutions including interdealer broker ICAP PLC have said they are under investigation for alleged manipulation of the ISDAfix rate.

Barclays confirmed the fine and said in a statement that the misconduct was "wholly incompatible with Barclays's purpose."

The $379 trillion market for interest-rate swaps is dominated by big banks, which charge everyone from hedge funds to manufacturing companies a premium for the trades designed to protect against fluctuations in their funding costs. The swaps are settled using the benchmark rate, which is known as ISDAFIX.

The ISDAFIX U.S. dollar rate is quoted twice a day at 11 a.m. and 3 p.m. Banks are polled about the rate during a specific time period and asked to submit estimates. Outliers are eliminated and an average is taken from the remaining contributions. During the polling window, the banks can change their contributions.

In a statement the CFTC said that Barclays, one of the panel banks which submitted rates that determined the daily U.S. ISDAfix rate, tried to artificially move the rate during the window when it is set.

This benefited the bank's trading position.

At the time Barclays didn't have any internal controls to regulate how the submission should be made, a situation the bank has now remedied.

Wednesday's fine comes as Barclays agreed to pay GBP1.5 billion ($2.33 billion) to a series of regulators to resolve charges that it manipulated foreign-exchange rates. In a statement the bank said it had put aside GBP2.05 billion to cover the fines.

In 2012 Barclays had entered into a nonprosecution agreement with the Justice Department after trying to rig the Libor benchmark rate. The DOJ said that despite trying to manipulate other rates Barclays didn't breach its nonprosecution agreement. Barclays said this was due to the significant "cultural and compliance changes" that the bank had put in place.

Write to Max Colchester at max.colchester@wsj.com

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