Symbian reported slower but steady growth during the first quarter of 2008, shipping 18.5 million units, up 16.5% from the same quarter last year. Its average royalty per unit was down to $4.10 from $4.50 year-over-year.

The software company, owned primarily by handset maker Nokia, said the number of Symbian handset models in the market during the quarter was up to 154 from 114 for the same quarter last year, and the number of Symbian handsets in development was 70 models, up slightly from 63 models for the same period one year earlier.

“We are pleased to report yet another significant milestone for Symbian,” said Nigel Clifford, Symbian CEO, in a statement. “With our customers shipping a total of 18.5 million Symbian mobile phones through over 250 major network operators globally in the quarter, we have now surpassed the 200-million cumulative shipment mark. In fact, by the end of the quarter, a cumulative total of 206 million Symbian mobile phones have been shipped by the world’s leading handset manufacturers, and each of our licensees has shipped a new Symbian mobile phone model since the beginning of 2008.”