ROTH IRA and ROTH 401k Contribution

Is it allowable for a participant in a 401k plan to max out his ROTH  deferrals in the plan, and also contribute to a ROTH IRA for the same year?

It would be unusual for someone to have enough income to be able to make a max ROTH-401(K) contribution and a max ROTH-IRA contribution but also have low enough income to qualify to make ROTH-IRA contributions. However, if you have someone who fits that specific fact pattern there is no prohibition against it.

Lou, I'm not so sure that is true.

I know several couples maxing out at least one 401k and one Roth IRA. While it may not be common, it certainly is not unusual.

It usally represents a 15% - 25% retirement savings rate. I maintained a 15% - 20% rate for most years. That is what allowed me to comfortably retire in my 50s.