I got my H1b on 27th Jan 2006. Before that I worked for 6 months on my OPT. I don't see any social security or medicare tax deducted from my salary on W2 form. What should I do in this case? Is this the employee or employer responsibility to pay these taxes ? I am getting conflicting answer for this.

Thanks.

You should NOT be paying Social Security and Medicare taxes while on OPT.

You SHOULD be paying once you start your H-1B visa. If you told your employer that you switched to H-1B visa and he failed to start withholding the tax, that is HIS problem, not yours.

The IRS position on this is clear: The EMPLOYER must pay the FICA taxes and withhold the employees' share from his salary. If the employer fails to do so, the IRS will hold the employer responsible for paying the tax. After 31 January 2007, the employer CANNOT LEGALLY do any back withholding of FICA taxes from the employee for tax year 2006. The employee can voluntarily agree to pay these taxes back to the employer (and get fired if he fails to agree), but the employer cannot go to court nor get help from the IRS to force the employee to pay these taxes if the employee does not agree to it.

If taxes are not being withheld in 2007, the employer can and should start backup withholding of FICA taxes retroactive to 1 January 2007.