This was done at the Frankfurt Bookfair 2009.
Publishing house "Eichborn" has a fly as their logo, so they got 200 real flies, attached a little banner to their legs and had them buzz around their stall at the fairground.
Banner had their claim "The publisher with the fly" and the location of their stall at the fair printed on it.
At the end of the clip it sais that "no fly was harmed in the process". Banners were attached with wax and fell off after a few hours.
Agency was Jung von Matt / Neckar