* Q3 EBITA 247.3 mln eur vs Rtrs poll estimate 252 mln eur

* Q3 revenue 500.9 mln eur vs Rtrs estimate 524 mln eur

* Group reiterates 2009 cost guidance of 1.28 bln eur (Adds details)

FRANKFURT, Nov 5 (Reuters) - German stock and derivatives market operator Deutsche Boerse (DB1Gn.DE: Quote, Profile, Research) reported a 36 percent fall in third-quarter operating profit late on Thursday, trailing market expectations as revenues shrank 12 percent.

The group, whose rivals include NYSE Euronext (NYX.N: Quote, Profile, Research) and CME Group (CME.O: Quote, Profile, Research) as well as Britain's London Stock Exchange (LSE.L: Quote, Profile, Research), posted earnings before interest, tax and amortisation (EBITA) of 243.7 million euros ($361.9 million) for the quarter to end-September, down from 385 million euros a year earlier.

Analysts had on average expected EBITA to fall 35 percent to 281 million euros, a Reuters poll showed. [ID:nLU093658].

Group revenue fell to 500.9 million euros due to lower trading activities in the group's Xetra and Eurex segments.

Analysts had on average seen revenues falling 15 percent to 524 million euros. ($1=.6734 Euro) (Reporting by Tyler Sitte, editing by Leslie Gevirtz)