From the 1st of September 2014 the Rome's overnight tax will be 3,5 euros per night per person, with the exception of children up to 10 years.
Will be asked to anyone staying in every overnight accommodation inside Rome’s boundaries.
The fee, not included in the total rate, must be paid cash at check in time.
Please note that the fee is also asked for to anyone who has booked or paid for the trip through a travel agency or tour operator and therefore pays by presenting a voucher at the reception desk.
source : http://www.sleepinitaly.com/overnighttax