Vanguard Group and Fidelity Investments manage hundreds of billions of dollars in defined-contribution money for thousands of retirement plans with millions of participants. Against competition like that, a foray into 401(k) plan management by a unit of robo-adviser Betterment, a company with $6.2 billion in assets under management, seems quixotic, to say the least.  

Betterment for Business, which began signing up corporate clients in January, announced today that it has more than 300 companies on board, with plan assets ranging from hundreds of thousands of dollars to more than $25 million. (It doesn't break out assets by business unit and won't give a number for total assets in the 401(k) unit.) Clients range from technology companies such as project management software designer Trello to mattress maker Casper and midsize professional service businesses such as law and architecture firms, as well as medical and dental practices, Betterment said. 

It may be good timing. The Department of Labor's fiduciary rule, which takes effect in April, requires advisers for retirement investments to put a client's interest before their own. A 2015 government report estimated that adviser conflicts of interest cost investors some $17 billion a year. The rule could be a regulatory catalyst for growth, said Tyler Cloherty, senior manager with Casey Quirk by Deloitte.