The HI surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed .
This analysis gives the radial variation of the lopsidedness in the HI spatial distribution .
The lopsidedness is quantified by the Fourier amplitude A _ { 1 } of the m = 1 component normalized to the average value .
It is also shown that in the radial region where the stellar disc and HI overlap , their A _ { 1 } coefficients are comparable .
All the galaxies studied show significant lopsidedness in HI .
The mean value of A _ { 1 } in the inner regions of the galaxies ( 1.5 - 2.5 scale lengths ) is \geq 0.2 .
This value of A _ { 1 } is twice the average value seen in the field galaxies .
Also , the lopsidedness is found to be smaller for late-type galaxies , this is opposite to the trend seen in the field galaxies .
These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies .
Further , a large fraction ( \sim 30 % ) shows a higher degree of lopsidedness ( A _ { 1 } \geq 0.3 ) .
It is also seen that the disk lopsidedness increases with the radius as demonstrated in earlier studies , but over a radial range that is two times larger than done in the previous studies .
The average lopsidedness of the halo potential is estimated to be \sim 10 % , assuming that the lopsidedness in HI disc is due to its response to the halo asymmetry .