Eight former employees of SpaceX say they were fired for speaking up against founder and chief executive Elon Musk, according to an unfair labour practice filing with a US board on Thursday.
The workers said they were let go for being part of a group that had drafted and circulated a letter to SpaceX executives in June.
The letter criticised Mr Musk, the world's richest person, and urged executives to make the company's culture more inclusive.
Reuters reported in June that SpaceX fired at least five employees who were involved in drafting the letter, which called Mr Musk a "distraction and embarrassment" to the company.
The charges filed on Wednesday with the National Labour Relations Board claim that SpaceX fired five employees the day after the letter was revealed, and four others in the two months after.
US labour law prohibits employers from firing workers who band together to push for better working conditions.
If the board finds that firings breach the law, it can order that workers be reinstated and given back-pay.
Mr Musk, also the head of electric-car maker Tesla, has been in the headlines over his $44 billion acquisition of Twitter and attempts to remake the social media company after he warned of a potential bankruptcy.
The New York Times reported this week that he has fired nearly two dozen employees at Twitter for openly criticising him. It is not yet clear if they will take legal action as a result.
The June letter sent to SpaceX executives focused on tweets Mr Musk had made since 2020, many of which were sexually suggestive.
It said his conduct did not align with the company's policies on diversity and workplace misconduct.
They called on SpaceX to publicly condemn Mr Musk's comments and to more clearly define the type of conduct prohibited by company policy.
Reuters contributed to this report