CME Group Inc. (CME) reclaimed its spot as the world's biggest derivatives exchange last month, topping Eurex in trading volume as interest rate speculation intensified.

Activity in CME's core rate futures products rose 8% from May to June as investors saw signs of improvement in the economic climate, fueling the busiest trading day this year for the Chicago-based exchange operator.

But overall trading volume remained well below year-ago levels at both CME and Eurex, and CME also saw its average rate per contract decline for the fourth straight month as pressure on fees mounted.

A total of 251 million contracts changed hands in June on CME's markets, while Eurex saw 232.5 million contracts traded, which includes the contribution from the International Securities Exchange options platform.

Average daily volume at CME came in at 11.4 million contracts, compared with 10.7 million at Eurex, co-owned by Deutsche Boerse (DB1.XE) and SWX Swiss Exchange.

Total volume remains 17% lower year-on-year at Eurex, and 20% lower for CME.

CME shares were 2.1% lower in early trade at $301.98, while Deutsche Boerse was also down at 52.04 euros.

CME on Thursday also reported a continued drop in its average rate per contract, which fell to 0.812 for the three-month period ended in May.

CME's average fee has fallen nearly 6 cents since January, when the exchange operator suffered its first RPC decline in more than a year.

Executives at CME have brushed off the decline, attributing it to a shift in the mix of products used by customers.

CME generally collects a higher fee for interest rate futures, where activity sagged as the Federal Reserve lowered its benchmark rate to near zero; equity index products, which have seen increased activity thanks to stock market volatility, carry a lower rate per contract.

Atlanta-based IntercontinentalExchange Inc. (ICE) reported a record average daily volume of 1.1 million contracts, a 14% increase year-on-year, as energy traders hedged against shifts in the price of crude oil.

China's move to increase oil reserves and Nigerian militants' raid on a Royal Dutch Shell PLC (RDSB.LN)-operated offshore oil platform helped drive the price of crude higher late in the month.

ICE also reported that its ICE Trust credit derivatives platform had cleared $1.3 trillion in notional contract value since its March launch.

-By Jacob Bunge, Dow Jones Newswires; 312-750-4117; jacob.bunge@dowjones.com; and Howard Packowitz, Dow Jones Newswires; 312-750-4132; howard.packowitz@dowjones.com