  We present results on ultra low noise YBa$_2$Cu$_3$O$_{7-\delta}$ nano
Superconducting QUantum Interference Devices (nanoSQUIDs). To realize such
devices, we implemented high quality YBCO nanowires, working as weak links
between two electrodes. We observe critical current modulation as a function of
an externally applied magnetic field in the full temperature range below the
transition temperature $T_C$. The white flux noise below 1 $\mu
\Phi_0/\sqrt{\mathrm{Hz}}$ at T = 8 K makes our nanoSQUIDs very attractive for
the detection of small spin systems.
