A new primary frequency standard, the atomic caesium fountain CSF1, has been put into operation at the Physikalisch-Technische Bundesanstalt (PTB). 133 Cs atoms are cooled in a magneto-optical trap and in optical molasses. They are launched to a height of 39 cm above the microwave cavity centre. The resulting Ramsey resonance signal has a full-width half-maximum linewidth (FWHM) of 0.88 Hz. The rst uncertainty evaluation yields a relative 1 frequency uncertainty of 1.4 10-15. The short-term relative frequency instability of the CSF1 for averaging time is typically 3.5 10-13 (/s)-1/2 , dictated by the available quartz oscillator as the local frequency source.