We report on direct writing of three-dimensional structures within the bulk of the Er3+-doped oxyfluoride precursor glass using tightly focused femtosecond Ti-sapphire laser pulses at 800 nm wavelength. Buried 1 µm diameter channel and spherical waveguides and three-dimensional periodic arrays of 1 µm diameter dots have been fabricated. In the latter case, the dot could be written with a single 25 fs laser pulse. When the laser pulse energy exceeded 10 µJ/pulse/µm2, the writing resulted in nano-ceraming of the precursor glass and creation of about 2 to 3 nm diameter Er3+-doped fluoride nanocrystals within the written dots/waveguides, as confirmed by a characteristic change in shape of the 1.5 µm photoluminescence band of the Er3+ ions. The diameter of the buried dots/wave-guides, and the photoinduced change of their refractive index, can be varied by changing the number of femtosecond laser pulses, the beam waist diameter, and/or laser intensity. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)