The ions produced by an external source and axially injected into the cyclotron are considered. The ion beam, with a matching structure, must be carefully placed on the orbit to be accelerated and well extracted. One has to take into account its position on the first turn, its shape, its chopped time structure due to RF (radio-frequency). A compromise between different necessities must be obtained. In fact beam losses can be significant, and the ion transfer efficiency from the source to the orbit varies from a few per cent to 70%!