Serrano died two days after Eduardo Castro Luque, another PRI member, was gunned down.
The 48-year-old was shot dead on Friday night outside his house in Ciudad Obregon, a city in the northern Mexican state of Sonora.
Authorities have suggested no link between the killings.
Mexican officials say an Institutional Revolutionary Party (PRI) legislator has been killed after an assailant stuck a knife into his chest.
Jaime Serrano Cedillo, a 45-year-old state deputy, was killed in Nezahualcoyotl city, situated about 11 kilometers (7 miles) east of the national capital Mexico City, on Sunday as he walked down a street, The Associated Press reported on Monday.
The attacker fled and no arrests have been made. Serrano was rushed to hospital by his family, but lost his life from the wounds.
"The Mexico state prosecutor's office will conduct the corresponding investigation to determine how the incidents occurred and find the whereabouts of the suspect or suspects, and will work in strict observance of the law," the Mexico state Attorney General's office said.