This paper deals about the design and the implementation of a very compact microstrip Tx/Rx diplexer for future UMTS mobile terminals. Miniaturized loop-resonators filters have been developed and combined INTO data_ds.tmp_pub_a diplexer structure, satisfying different constraints such as a high electrical performance level (especially, a very narrow bandwidth, low insertion losses and a high out-of-band rejection), or also a great compactness and a compatibility with a standard planar integration.