We present Giant Metrewave Radio Telescope ( GMRT ) observations for three ( viz. , DDO 68 , SDSS J2104–0035 and UGC 772 ) of the six most metal-deficient actively star-forming galaxies known .
Although there is a debate as to whether these galaxies are undergoing their first episode of star formation or not , they are ‘ young ’ in the sense that their ISM is chemically unevolved .
In this regard , they are the nearest equivalents of young galaxies in the early Universe .

All three galaxies , that we have observed , have irregular \HI morphologies and kinematics , which we interpret as either due to tidal interaction with neighbouring galaxies , or the consequences of a recent merger .
The remaining three of the six most metal-deficient galaxies are also known to have highly disturbed \HI distributions and are interacting .
It is interesting because these galaxies were chosen solely on the basis of their metallicity and not for any particular signs of interaction .
In this sense ( i.e. , their gas has not yet had time to settle into a regular disc ) , one could regard these extremely metal deficient ( XMD ) galaxies as ‘ young ’ .
The current star formation episode is likely to have been triggered by interaction/merger .
It is also possible that the tidal interaction has lead to enhanced mixing with metal-poor gas in outer disc , and hence to a low gas-phase metallicity in the central star-forming regions .

We also try to determine the threshold gas-density for star-formation in our sample of galaxies , and find that in general these galaxies do not show a one-to-one correspondence between regions of high \HI column density and regions with current star formation .
However , to the extent that one can define a threshold density , its value ( \sim 10 ^ { 21 } atoms cm ^ { -2 } ) is similar to that in galaxies with much higher metallicity .
The highest column densities that we detect in regions far outside star-forming regions ( i.e. , a lower limit to the star-formation threshold ) are \sim 2 \times 10 ^ { 21 } atoms cm ^ { -2 } .